**Problem:**
Production error showed runs being marked as failed with stop_reason=None,
which violates LettaStopReason's Pydantic schema (requires valid enum value).
This caused cascading validation errors that got stored in metadata.
Example error:
```
Run is already in a terminal state failed with stop reason None, but is being
updated with data {'status': 'failed', 'stop_reason': None, 'metadata':
{'error': "1 validation error for LettaStopReason\nstop_reason Input should
be 'end_turn', 'error', ... [type=enum, input_value=None]"}}
```
**Root Causes:**
1. routers/v1/agents.py had 3 exception handlers creating RunUpdate(status=failed)
without stop_reason
2. Success path assumed result.stop_reason always exists (AttributeError if None)
3. run_manager.py tried to create LettaStopReason(stop_reason=None) when
refreshing result messages
**Fixes:**
1. Added stop_reason=StopReasonType.error to 3 exception handlers
2. Added defensive None checks before accessing result.stop_reason.stop_reason
3. Added fallback to StopReasonType.error when pydantic_run.stop_reason is None
**Trigger:**
OpenAI BadRequestError for invalid tool schema → exception handlers marked
run as failed without stop_reason → validation error when constructing response
👾 Generated with [Letta Code](https://letta.com)
Co-authored-by: Letta <noreply@letta.com>
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