fix(rmcp): surface JSON-RPC error bodies on HTTP 4xx responses

[kakarot-dev]

When a server returns a 4xx status with Content-Type: application/json, attempt to deserialize the body as a ServerJsonRpcMessage before falling back to UnexpectedServerResponse. This allows JSON-RPC error payloads carried on HTTP error responses to be surfaced as McpError instead of being lost in a transport-level error string.

Fixes#724

Attempt to deserialize HTTP 4xx response bodies as ServerJsonRpcMessage when Content-Type: application/json, surfacing JSON-RPC errors as McpError instead of discarding them as UnexpectedServerResponse.

How Has This Been Tested?

Tested locally with a mock axum server against three scenarios: valid JSON-RPC error body on 400, non-JSON 400, and malformed JSON on 400. All pass.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

I created a test file locally, Test file available if needed

[DaleSeo]

Thanks for tackling this, @kakarot-dev! I have a couple of comments below. It looks like you've done some manual testing, but it would be great if you could add some automated tests to avoid any regressions.