fix: increase branch_detail max_tokens to 8192 and add response logging

Truncated output at 4096 tokens produces invalid JSON mid-generation.
Also logs stop_reason and output_tokens per attempt to diagnose failures.

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

backend/app/core/ai_tree_generator_service.py

@@ -202,14 +202,23 @@ async def generate_branch_detail(
     for attempt in range(2):
         response = await client.messages.create(
             model=settings.AI_MODEL,
-            max_tokens=4096,
+            max_tokens=8192,
             system=BRANCH_DETAIL_SYSTEM_PROMPT,
             messages=messages,
         )
 
-        raw_text = _strip_markdown_fences(response.content[0].text)
         total_input += response.usage.input_tokens
         total_output += response.usage.output_tokens
+        logger.debug(
+            "branch_detail attempt=%d stop_reason=%s content_blocks=%d output_tokens=%d",
+            attempt,
+            response.stop_reason,
+            len(response.content),
+            response.usage.output_tokens,
+        )
+        raw_text = _strip_markdown_fences(response.content[0].text) if response.content else ""
+        if not raw_text:
+            logger.warning("branch_detail attempt=%d returned empty text, stop_reason=%s", attempt, response.stop_reason)
 
         try:
             branch_tree = json.loads(raw_text)