feat: surface structured rejection fields from gRPC response (RFC-008 B8) (#61)

* feat: surface structured rejection fields from gRPC response (RFC-008 B8)

- Regenerate mcp_pb2.py from updated proto with error_code,
  rejection_detail, requested_capability, presented_capability
  fields (15-18) and SCOPE_INSUFFICIENT enum value (9)
- Add SCOPE_INSUFFICIENT to deny_reason_map in evaluate_tool_access()
- Return error_code, requested_capability, presented_capability
  in the evaluate_tool_access() result dict
- Restore try/except TypeError handler for descriptor pool collisions

* fix: address Copilot review feedback on RFC-008 B8

- Remove duplicated import block in mcp_pb2.py
- Remove grpcio>=1.76.0 version gate from mcp_pb2_grpc.py (SDK
  supports grpcio>=1.60.0)
- Remove unused warnings import from mcp_pb2_grpc.py
- Add rejection_detail to evaluate_tool_access() return dict
- Add unit tests for structured rejection fields

Author: beonde

capiscio_sdk/_rpc/client.py

@@ -1607,6 +1607,7 @@ def evaluate_tool_access(
             mcp_pb2.MCP_DENY_REASON_TOOL_NOT_ALLOWED: "tool_not_allowed",
             mcp_pb2.MCP_DENY_REASON_ISSUER_UNTRUSTED: "issuer_untrusted",
             mcp_pb2.MCP_DENY_REASON_POLICY_DENIED: "policy_denied",
+            mcp_pb2.MCP_DENY_REASON_SCOPE_INSUFFICIENT: "scope_insufficient",
         }
 
         auth_level_map = {
@@ -1636,6 +1637,10 @@ def evaluate_tool_access(
             "evidence_json": response.evidence_json,
             "evidence_id": response.evidence_id,
             "timestamp": timestamp_str,
+            "error_code": response.error_code,
+            "rejection_detail": response.rejection_detail,
+            "requested_capability": response.requested_capability,
+            "presented_capability": response.presented_capability,
         }
 
     def verify_server_identity(