test: add cross-tenant isolation tests for Task 6 UUID audit

Tests cover:
- Tree GET/PUT returns 404 for cross-account access
- Session GET returns 404 for cross-user access
- AI session GET returns 404 for cross-user access
- AI session retry-psa-push requires ownership
- Upload URL returns 404 for cross-account access
- Share revoke returns 404 for cross-user access

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_tenant_isolation_p0.py

@@ -212,3 +212,197 @@ async def test_ai_session_search_cannot_see_other_users_sessions(
     assert str(session_b.id) not in ids, (
         "User A can see User B's session via search — cross-user leak within account"
     )
+
+
+# ── Task 6: Cross-tenant UUID audit ─────────────────────────────────────────
+
+
+@pytest.mark.asyncio
+async def test_get_tree_returns_404_not_403_for_other_account(
+    client: AsyncClient, test_db: AsyncSession
+):
+    """Account A gets 404 (not 403) when accessing Account B's private tree."""
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "t6-tree-a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "t6-tree-b")
+    tree_b = await _create_private_tree(test_db, acct_b, user_b)
+    await test_db.commit()
+
+    headers_a = await _login(client, user_a.email, pass_a)
+    resp = await client.get(f"/api/v1/trees/{tree_b.id}", headers=headers_a)
+    assert resp.status_code == 404, (
+        f"Expected 404 for cross-tenant tree access, got {resp.status_code}"
+    )
+
+
+@pytest.mark.asyncio
+async def test_update_tree_returns_404_not_403_for_other_account(
+    client: AsyncClient, test_db: AsyncSession
+):
+    """Account A gets 404 (not 403) when trying to update Account B's tree."""
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "t6-upd-a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "t6-upd-b")
+    tree_b = await _create_private_tree(test_db, acct_b, user_b)
+    await test_db.commit()
+
+    headers_a = await _login(client, user_a.email, pass_a)
+    resp = await client.put(
+        f"/api/v1/trees/{tree_b.id}",
+        json={"name": "Hacked"},
+        headers=headers_a,
+    )
+    assert resp.status_code == 404, (
+        f"Expected 404 for cross-tenant tree update, got {resp.status_code}"
+    )
+
+
+@pytest.mark.asyncio
+async def test_get_session_returns_404_not_403_for_other_user(
+    client: AsyncClient, test_db: AsyncSession
+):
+    """User A gets 404 (not 403) when accessing User B's session."""
+    from app.models.session import Session
+
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "t6-sess-a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "t6-sess-b")
+    tree_b = await _create_private_tree(test_db, acct_b, user_b)
+
+    session_b = Session(
+        tree_id=tree_b.id,
+        user_id=user_b.id,
+        tree_snapshot={"id": "root", "type": "start", "children": []},
+        path_taken=[],
+        decisions=[],
+    )
+    test_db.add(session_b)
+    await test_db.commit()
+
+    headers_a = await _login(client, user_a.email, pass_a)
+    resp = await client.get(f"/api/v1/sessions/{session_b.id}", headers=headers_a)
+    assert resp.status_code == 404, (
+        f"Expected 404 for cross-user session access, got {resp.status_code}"
+    )
+
+
+@pytest.mark.asyncio
+async def test_ai_session_get_returns_404_not_403_for_other_user(
+    client: AsyncClient, test_db: AsyncSession
+):
+    """User A gets 404 (not 403) when accessing User B's AI session."""
+    from app.models.ai_session import AISession
+
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "t6-ais-a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "t6-ais-b")
+
+    ai_session_b = AISession(
+        user_id=user_b.id,
+        account_id=acct_b.id,
+        problem_summary="Test session",
+        problem_domain="networking",
+        status="active",
+    )
+    test_db.add(ai_session_b)
+    await test_db.commit()
+
+    headers_a = await _login(client, user_a.email, pass_a)
+    resp = await client.get(f"/api/v1/ai-sessions/{ai_session_b.id}", headers=headers_a)
+    assert resp.status_code == 404, (
+        f"Expected 404 for cross-user AI session access, got {resp.status_code}"
+    )
+
+
+@pytest.mark.asyncio
+async def test_ai_session_retry_psa_push_requires_ownership(
+    client: AsyncClient, test_db: AsyncSession
+):
+    """User A cannot retry PSA push for User B's AI session."""
+    from app.models.ai_session import AISession
+
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "t6-psa-a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "t6-psa-b")
+
+    ai_session_b = AISession(
+        user_id=user_b.id,
+        account_id=acct_b.id,
+        problem_summary="PSA test",
+        problem_domain="networking",
+        status="resolved",
+    )
+    test_db.add(ai_session_b)
+    await test_db.commit()
+
+    headers_a = await _login(client, user_a.email, pass_a)
+    resp = await client.post(
+        f"/api/v1/ai-sessions/{ai_session_b.id}/retry-psa-push",
+        headers=headers_a,
+    )
+    assert resp.status_code == 404, (
+        f"Expected 404 for cross-user retry-psa-push, got {resp.status_code}"
+    )
+
+
+@pytest.mark.asyncio
+async def test_upload_url_returns_404_not_403_for_other_account(
+    client: AsyncClient, test_db: AsyncSession
+):
+    """User A gets 404 (not 403) when accessing User B's upload URL."""
+    from app.models.file_upload import FileUpload
+
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "t6-upl-a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "t6-upl-b")
+
+    upload_b = FileUpload(
+        account_id=acct_b.id,
+        uploaded_by=user_b.id,
+        filename="secret.png",
+        content_type="image/png",
+        size_bytes=1024,
+        storage_key="test/secret.png",
+    )
+    test_db.add(upload_b)
+    await test_db.commit()
+
+    headers_a = await _login(client, user_a.email, pass_a)
+    resp = await client.get(f"/api/v1/uploads/{upload_b.id}/url", headers=headers_a)
+    assert resp.status_code in (404, 503), (
+        f"Expected 404 (or 503 if storage not configured) for cross-account upload, got {resp.status_code}"
+    )
+
+
+@pytest.mark.asyncio
+async def test_share_revoke_returns_404_not_403_for_other_user(
+    client: AsyncClient, test_db: AsyncSession
+):
+    """User A gets 404 (not 403) when revoking User B's share."""
+    from app.models.session import Session
+    from app.models.session_share import SessionShare
+
+    acct_a, user_a, pass_a = await _create_account_and_user(test_db, "t6-shr-a")
+    acct_b, user_b, pass_b = await _create_account_and_user(test_db, "t6-shr-b")
+    tree_b = await _create_private_tree(test_db, acct_b, user_b)
+
+    session_b = Session(
+        tree_id=tree_b.id,
+        user_id=user_b.id,
+        tree_snapshot={"id": "root", "type": "start", "children": []},
+        path_taken=[],
+        decisions=[],
+    )
+    test_db.add(session_b)
+    await test_db.flush()
+
+    share_b = SessionShare(
+        session_id=session_b.id,
+        account_id=acct_b.id,
+        share_token="test-token-unique-" + uuid.uuid4().hex[:8],
+        share_name="Test",
+        visibility="public",
+        created_by=user_b.id,
+    )
+    test_db.add(share_b)
+    await test_db.commit()
+
+    headers_a = await _login(client, user_a.email, pass_a)
+    resp = await client.delete(f"/api/v1/shares/{share_b.id}", headers=headers_a)
+    assert resp.status_code == 404, (
+        f"Expected 404 for cross-user share revoke, got {resp.status_code}"
+    )