fix: return 400 instead of 500 for image fetch errors (#9157)

**Problem:**
When a user sends a message with an image URL that times out or fails to
fetch, the server returns a 500 Internal Server Error with a generic message.
This is confusing because the user doesn't know what went wrong.

**Root Cause:**
`LettaImageFetchError` was not registered in the exception handlers, so it
bubbled up as an unhandled exception.

**Fix:**
Register `LettaImageFetchError` with the 400 Bad Request handler. Now users
get a clear error message like:
```
Failed to fetch image from https://...: Timeout after 2 attempts
```

This tells users exactly what went wrong so they can retry with a different
image or verify the URL is accessible.

👾 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

letta/server/rest_api/app.py

@@ -38,6 +38,7 @@ from letta.errors import (
     HandleNotFoundError,
     LettaAgentNotFoundError,
     LettaExpiredError,
+    LettaImageFetchError,
     LettaInvalidArgumentError,
     LettaInvalidMCPSchemaError,
     LettaMCPConnectionError,
@@ -477,6 +478,7 @@ def create_application() -> "FastAPI":
     app.add_exception_handler(LettaToolNameConflictError, _error_handler_400)
     app.add_exception_handler(AgentFileImportError, _error_handler_400)
     app.add_exception_handler(EmbeddingConfigRequiredError, _error_handler_400)
+    app.add_exception_handler(LettaImageFetchError, _error_handler_400)
     app.add_exception_handler(ValueError, _error_handler_400)
 
     # 404 Not Found errors