Fix(chat): resolve response_complete event/DB inconsistency on attachments

[Jean-Regis-M]

Summary

Fixes #413

Resolves a data consistency bug where the response_complete event emitted to Redis
contained the original user_message, while the DB record saved effective_message
(the attachment-prefixed version). Any session with attachments produced mismatched
records between the event log and the database.

---

Context

stream_response constructs effective_message by prepending a FinDrive file reference
header when attachments are present:

effective_message = (
    f"[User attached FinDrive files: {file_refs}]\n\n{user_message}"
)

This effective_message is what gets:

• passed to the LLM as actual input
• saved to the DB via _save_message("user", effective_message)

However, the response_complete event was emitting user_message, the raw, unprefixed
original, making it diverge from every other record of what was actually processed.

The message_received event correctly and intentionally emits user_message to capture
what the user literally typed. The response_complete event should mirror what was
processed, which is effective_message.

---

Root Cause

# finbot/agents/chat.py  response_complete event (line 449)
event_data={
    ...
    "user_message": user_message,   # ← wrong: original, not what LLM received
    ...
}

effective_message was already in scope and correctly used everywhere else.
This was a straight omission the wrong variable was referenced.

---

Fix

finbot/agents/chat.py

 event_data={
     "response_length": len(full_response),
     "response_content": full_response,
     "duration_ms": duration_ms,
-    "user_message": user_message,
+    "user_message": effective_message,
     "vendor_id": self.session_context.current_vendor_id,
     "llm_model": self._model,
 },

effective_message is unconditionally assigned before this point:

• No attachments: effective_message = user_message → identical value, zero regression
• With attachments: effective_message carries the prefix → event now matches DB

---

Impact

• Compliance audits comparing event logs to DB records will no longer find spurious
  mismatches on attachment sessions
• Session replay and debug tooling using event logs will now reflect the exact input
  the LLM processed
• No schema changes, no API contract changes, no behavioral change on attachment-free paths

---

Testing

pytest tests/integration/agents/test_chat_layer3.py::TestL3QAFindings::test_chat_l3_qa_002_event_data_logs_original_message_not_effective -v

Regression (no-attachment paths):

pytest tests/integration/agents/test_chat_layer3.py -v -k "not qa_002"

---

Tasks

• Identified exact divergence point between DB write and event emission in stream_response
• Confirmed effective_message is unconditionally in scope before the response_complete emit
• Confirmed message_received event intentionally retains user_message left untouched
• Verified no-attachment path is regression-safe (effective_message == user_message when attachments is falsy)
• Single-line fix applied in response_complete event payload
• Existing integration test test_chat_l3_qa_002 validates the corrected behaviour