fix(llm): log and handle non-STOP finish_reason from Gemini

When Gemini stops generation early (e.g. due to safety filters or
thinking-model quirks), finish_reason != STOP but no exception is raised,
causing the caller to receive truncated JSON and a confusing 502 "invalid
JSON" error. Now:
- finish_reason is extracted from candidates[0] and stored in ai_call_logs
- any non-STOP finish_reason raises HTTP 502 with a clear message
- Alembic migration adds the finish_reason column to ai_call_logs

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

backend/alembic/versions/b2c3d4e5f6a1_add_finish_reason_to_ai_call_logs.py

@@ -0,0 +1,29 @@
+"""add_finish_reason_to_ai_call_logs
+
+Revision ID: b2c3d4e5f6a1
+Revises: a1b2c3d4e5f6
+Create Date: 2026-03-01 00:00:00.000000
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+
+from alembic import op
+
+revision: str = "b2c3d4e5f6a1"
+down_revision: Union[str, None] = "a1b2c3d4e5f6"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "ai_call_logs",
+        sa.Column("finish_reason", sa.Text(), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("ai_call_logs", "finish_reason")

backend/innercontext/llm.py

@@ -47,11 +47,22 @@ def call_gemini(
             user_input = str(contents)
 
     start = time.monotonic()
-    success, error_detail, response = True, None, None
+    success, error_detail, response, finish_reason = True, None, None, None
     try:
         response = client.models.generate_content(
             model=model, contents=contents, config=config
         )
+        with suppress(Exception):
+            finish_reason = response.candidates[0].finish_reason.name
+        if finish_reason and finish_reason != "STOP":
+            success = False
+            error_detail = f"finish_reason: {finish_reason}"
+            raise HTTPException(
+                status_code=502,
+                detail=f"Gemini stopped early (finish_reason={finish_reason})",
+            )
+    except HTTPException:
+        raise
     except Exception as exc:
         success = False
         error_detail = str(exc)
@@ -81,6 +92,7 @@ def call_gemini(
                     else None
                 ),
                 duration_ms=duration_ms,
+                finish_reason=finish_reason,
                 success=success,
                 error_detail=error_detail,
             )

backend/innercontext/models/ai_log.py

@@ -23,5 +23,6 @@ class AICallLog(SQLModel, table=True):
     completion_tokens: int | None = Field(default=None)
     total_tokens: int | None = Field(default=None)
     duration_ms: int | None = Field(default=None)
+    finish_reason: str | None = Field(default=None)
     success: bool = Field(default=True, index=True)
     error_detail: str | None = Field(default=None)