feat: 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

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

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

llama_stack/distribution/access_control/access_control.py

@@ -106,4 +106,21 @@ def is_action_allowed(
 
 
 class AccessDeniedError(RuntimeError):
-    pass
+    def __init__(self, action: str | None = None, resource: ProtectedResource | None = None, user: User | None = None):
+        self.action = action
+        self.resource = resource
+        self.user = user
+
+        # Build detailed error message
+        if action and resource and user:
+            resource_info = f"{resource.type}::{resource.identifier}"
+            user_info = f"'{user.principal}'"
+            if user.attributes:
+                attrs = ", ".join([f"{k}={v}" for k, v in user.attributes.items()])
+                user_info += f" (attributes: {attrs})"
+
+            message = f"User {user_info} cannot perform action '{action}' on resource '{resource_info}'"
+        else:
+            message = "Insufficient permissions"
+
+        super().__init__(message)