fix: AccessDeniedError leads to HTTP 500 instead of error 403 (#2595)

Resolves access control error visibility issues where 500 errors were
returned instead of proper 403 responses with actionable error messages.

• Enhance AccessDeniedError with detailed context and improve exception
handling
• Enhanced AccessDeniedError class to include user, action, and resource
context
  - Added constructor parameters for action, resource, and user
- Generate detailed error messages showing user principal, attributes,
and attempted resource
- Backward compatible with existing usage (falls back to generic
message)

• Updated exception handling in server.py
  - Import AccessDeniedError from access_control module
  - Return proper 403 status codes with detailed error messages
- Separate handling for PermissionError (generic) vs AccessDeniedError
(detailed)

• Enhanced error context at raise sites
- Updated routing_tables/common.py to pass action, resource, and user
context
- Updated agents persistence to include context in access denied errors
  - Provides better debugging information for access control issues

• Added comprehensive unit tests
  - Created tests/unit/server/test_server.py with 13 test cases
  - Covers AccessDeniedError with and without context
- Tests all exception types (ValidationError, BadRequestError,
AuthenticationRequiredError, etc.)
  - Validates proper HTTP status codes and error message formats


# What does this PR do?
<!-- Provide a short summary of what this PR does and why. Link to
relevant issues if applicable. -->

<!-- If resolving an issue, uncomment and update the line below -->
<!-- Closes #[issue-number] -->

## Test Plan

```
server:
  port: 8321
    access_policy:
    - permit:
        principal: admin
        actions: [create, read, delete]
        when: user with admin in groups
    - permit:
        actions: [read]
        when: user with system:authenticated in roles
```
then:

```
curl --request POST --url http://localhost:8321/v1/vector-dbs \
  --header "Authorization: Bearer your-bearer" \
  --data '{
    "vector_db_id": "my_demo_vector_db",
    "embedding_model": "ibm-granite/granite-embedding-125m-english",
    "embedding_dimension": 768,
    "provider_id": "milvus"
  }'
 
```

depending if user is in group admin or not, you should get the
`AccessDeniedError`. Before this PR, this was leading to an error 500
and `Traceback` displayed in the logs.
After the PR, logs display a simpler error (unless DEBUG logging is set)
and a 403 Forbidden error is returned on the HTTP side.

---------

Signed-off-by: Akram Ben Aissi <<akram.benaissi@gmail.com>>

llama_stack/distribution/server/server.py

@@ -9,6 +9,7 @@ import asyncio
 import functools
 import inspect
 import json
+import logging
 import os
 import ssl
 import sys
@@ -31,6 +32,7 @@ from openai import BadRequestError
 from pydantic import BaseModel, ValidationError
 
 from llama_stack.apis.common.responses import PaginatedResponse
+from llama_stack.distribution.access_control.access_control import AccessDeniedError
 from llama_stack.distribution.datatypes import AuthenticationRequiredError, LoggingConfig, StackRunConfig
 from llama_stack.distribution.distribution import builtin_automatically_routed_apis
 from llama_stack.distribution.request_headers import PROVIDER_DATA_VAR, User, request_provider_data_context
@@ -116,7 +118,7 @@ def translate_exception(exc: Exception) -> HTTPException | RequestValidationErro
         return HTTPException(status_code=400, detail=f"Invalid value: {str(exc)}")
     elif isinstance(exc, BadRequestError):
         return HTTPException(status_code=400, detail=str(exc))
-    elif isinstance(exc, PermissionError):
+    elif isinstance(exc, PermissionError | AccessDeniedError):
         return HTTPException(status_code=403, detail=f"Permission denied: {str(exc)}")
     elif isinstance(exc, asyncio.TimeoutError | TimeoutError):
         return HTTPException(status_code=504, detail=f"Operation timed out: {str(exc)}")
@@ -236,7 +238,10 @@ def create_dynamic_typed_route(func: Any, method: str, route: str) -> Callable:
                         result.url = route
                     return result
             except Exception as e:
-                logger.exception(f"Error executing endpoint {route=} {method=}")
+                if logger.isEnabledFor(logging.DEBUG):
+                    logger.exception(f"Error executing endpoint {route=} {method=}")
+                else:
+                    logger.error(f"Error executing endpoint {route=} {method=}: {str(e)}")
                 raise translate_exception(e) from e
 
     sig = inspect.signature(func)