fix(flowpilot): fix session detail 500 — build AISessionDetail manually

Root cause: AISessionDetail.model_validate(session) tried to validate the
ORM relationship 'steps' which has field 'id', but AISessionStepResponse
expects 'step_id'. This caused a Pydantic ValidationError on every session
detail load.

Fix: Construct AISessionDetail manually from ORM fields, passing the
already-built step_responses list directly instead of relying on
model_validate to serialize the ORM steps relationship.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/endpoints/ai_sessions.py

@@ -645,16 +645,33 @@ async def get_session(
             confidence_score=step.confidence_at_step,
         ))
 
-    try:
-        detail = AISessionDetail.model_validate(session)
-        detail.steps = step_responses
-        return detail
-    except Exception as e:
-        logger.exception("Failed to serialize session %s: %s", session_id, e)
-        raise HTTPException(
-            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
-            detail=f"Session serialization error: {type(e).__name__}: {str(e)[:200]}",
-        )
+    # Build detail manually — AISessionDetail.model_validate(session) fails because
+    # the ORM relationship 'steps' has 'id' not 'step_id', causing validation errors.
+    # Instead, extract non-step fields from ORM and set step_responses separately.
+    detail = AISessionDetail(
+        id=session.id,
+        status=session.status,
+        intake_type=session.intake_type,
+        intake_content=session.intake_content or {},
+        problem_summary=session.problem_summary,
+        problem_domain=session.problem_domain,
+        confidence_tier=session.confidence_tier,
+        step_count=session.step_count,
+        session_rating=session.session_rating,
+        psa_ticket_id=session.psa_ticket_id,
+        psa_connection_id=session.psa_connection_id,
+        escalation_reason=session.escalation_reason,
+        matched_flow_id=session.matched_flow_id,
+        match_score=getattr(session, 'match_score', None),
+        resolution_summary=session.resolution_summary,
+        resolution_action=getattr(session, 'resolution_action', None),
+        session_feedback=session.session_feedback,
+        ticket_data=session.ticket_data,
+        created_at=session.created_at,
+        resolved_at=session.resolved_at,
+        steps=step_responses,
+    )
+    return detail
 
 
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