fix: type structured MCP call errors

[kiwigitops]

Fixes #1568.

Summary:

• add a Responses-specific ResponseMcpCallError object type for MCP tool failures
• allow mcp_call.error to be either the existing string value or a structured error object
• add a regression for the reported mcp_protocol_error payload

Verification:

• jest tests/responsesItems.test.ts --runInBand
• prettier --check src/resources/responses/responses.ts tests/responsesItems.test.ts
• eslint src/resources/responses/responses.ts tests/responsesItems.test.ts
• tsc --noEmit --pretty false
• git diff --check

[kiwigitops]

Quick bump on this. Happy to adjust the type shape or test coverage if you'd prefer a different approach.