fix(core): return 400 for ContextWindowExceededError instead of 500 (#9348)

SystemPromptTokenExceededError (a subclass of ContextWindowExceededError)
was hitting the generic 500 handler in Datadog. This is a client-side
configuration issue (system prompt too large for context window), so it
should return 400 Bad Request.

Datadog: https://us5.datadoghq.com/error-tracking/issue/3360e252-d5f1-11f0-8467-da7ad0900000
Also covers: https://us5.datadoghq.com/error-tracking/issue/6e50c57c

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

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

letta/server/rest_api/app.py

@@ -33,6 +33,7 @@ from letta.errors import (
     AgentNotFoundForExportError,
     BedrockPermissionError,
     ConcurrentUpdateError,
+    ContextWindowExceededError,
     ConversationBusyError,
     EmbeddingConfigRequiredError,
     HandleNotFoundError,
@@ -513,6 +514,7 @@ def create_application() -> "FastAPI":
     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(ContextWindowExceededError, _error_handler_400)
     app.add_exception_handler(ValueError, _error_handler_400)
 
     # 404 Not Found errors