MCP: provide sample request payload during error

Per discussion from https://github.com/apache/polaris-tools/pull/89, it would be helpful "if the error message pointed to the expected shape (e.g., required fields or a small example) to help callers fix issues quickly"

Start by reading the discussion in pull request #89 and tracing the MCP error path that handles invalid request payloads. No file or test is named in the issue; done means callers see the expected request shape, including required fields or a small example, in the error message.

Written by the indexing model from the issue text.