fix(BA-6003): restore indent=2 JSON dump in coordinator exception log

The dual-catch unification accidentally degraded the log payload from
ex.json(indent=2) (the ValidationError-only path) to str(ex)
(the unified path). Both BackendAISchemaValidationFailed and
ValidationError expose errors(), so we can keep the indented
JSON shape via json.dumps(ex.errors(), indent=2, default=str).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jopemachine

src/ai/backend/appproxy/coordinator/server.py

@@ -6,6 +6,7 @@
 import importlib
 import importlib.resources
 import ipaddress
+import json
 import logging
 import os
 import pwd
@@ -200,7 +201,10 @@ async def exception_middleware(
     except (BackendAISchemaValidationFailed, ValidationError) as ex:
         # ``ValidationError`` covers plain ``BaseModel`` subclasses that
         # skip the ``BackendAISchema`` auto-conversion override.
-        log.exception("Failed to create response model: {}", ex)
+        log.exception(
+            "Failed to create response model: {}",
+            json.dumps(ex.errors(), indent=2, default=str),
+        )
         raise InternalServerError() from ex
     except BackendAIError as ex:
         if ex.status_code == 500: