feat: add credit verification before agent message endpoints [LET-XXXX] (#9433)

* feat: add credit verification before agent message endpoints

Add credit verification checks to message endpoints to prevent
execution when organizations have insufficient credits.

- Add InsufficientCreditsError exception type
- Add CreditVerificationService that calls step-orchestrator API
- Add credit checks to /agents/{id}/messages endpoints
- Add credit checks to /conversations/{id}/messages endpoint

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

Co-Authored-By: Letta <noreply@letta.com>

* surface error in ade

* do per step instead

* parallel check

* parallel to step

* small fixes

* stage publish api

* fixes

* revert unnecessary frontend changes

* insufficient credits stop reason

---------

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

letta/errors.py

@@ -451,6 +451,16 @@ class AgentFileImportError(Exception):
     """Exception raised during agent file import operations"""
 
 
+class InsufficientCreditsError(LettaError):
+    """Raised when an organization has no remaining credits."""
+
+    def __init__(self):
+        super().__init__(
+            message="Insufficient credits to process this request.",
+            details={"error_code": "INSUFFICIENT_CREDITS"},
+        )
+
+
 class RunCancelError(LettaError):
     """Error raised when a run cannot be cancelled."""