managed-chatkit: normalize upstream error payloads to readable strings

[shuofengzhang]

What changed

• Added resolve_error_message(...) in managed-chatkit/backend/app/main.py to normalize upstream error payloads into a readable string.
• Updated /api/create-session error handling to use this resolver instead of directly forwarding payload["error"].
• Added regression tests in managed-chatkit/backend/tests/test_main.py covering:
  • nested upstream error objects ({"error": {"message": ...}})
  • plain string errors ({"error": "..."})
  • endpoint behavior ensuring JSON response always returns a string error field.

Why

The OpenAI-style error shape is typically nested (for example, {"error": {"message": "..."}}).
Previously, the backend forwarded payload["error"] directly, which could return an object instead of a string.

That makes frontend/debug output inconsistent and can produce low-signal error displays (for example object rendering instead of a clear message).

Practical gain / Why this matters

• Immediate user-facing reliability improvement: /api/create-session now consistently returns a human-readable string error.
• Lower debugging friction: integrators see precise failure messages from upstream rather than opaque object payloads.
• Maintainer safety: the change is tightly scoped to error-message extraction and keeps status codes/cookie behavior unchanged.

Insight / Why this matters

• Root cause: upstream API errors are often structured objects, but previous code assumed error was already a string.
• Why easy to miss: success paths and simple error paths work; object-shaped errors appear only in specific failure responses.
• Tradeoff: this intentionally prefers readable message extraction over forwarding full raw error objects in this endpoint.
• Impact: avoids ambiguous client error rendering and improves operational clarity during integration failures.

Testing

• ✅ cd managed-chatkit/backend && source .venv/bin/activate && pytest -q
  • 3 passed
• ✅ cd managed-chatkit/backend && source .venv/bin/activate && python -m ruff check app tests
• ✅ cd managed-chatkit/backend && source .venv/bin/activate && python -m ruff format --check app tests
• ✅ cd managed-chatkit/backend && source .venv/bin/activate && python -m mypy app --ignore-missing-imports

[m74278803-cmyk]

shuofengzhang:fix-managed-chatkit-error-message-shape

[m74278803-cmyk]

shuofengzhang:fix-managed-chatkit-error-message-shape