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/providers/inline/agents/meta_reference/persistence.py

@@ -53,7 +53,7 @@ class AgentPersistence:
             identifier=name,  # should this be qualified in any way?
         )
         if not is_action_allowed(self.policy, "create", session_info, user):
-            raise AccessDeniedError()
+            raise AccessDeniedError("create", session_info, user)
 
         await self.kvstore.set(
             key=f"session:{self.agent_id}:{session_id}",