Echo the JSON-RPC request id in MCP errors

What does this MR do and why?

The /api/v4/mcp and /api/v4/orbit/mcp endpoints returned a null id in error responses, even when the request had a readable id. JSON-RPC 2.0 says the error id must match the request id. Without this, a client with several requests in flight cannot tell which call an error belongs to.

Both endpoints now use a small jsonrpc_request_id helper. The helper returns the id only when it is an Integer or a String. It returns null for any other id, as JSON-RPC requires.

Measured before and after, on the same requests:

Request	Endpoint	Before	After
Bad protocol version, id "abc-123", error -32602	/api/v4/mcp	null	"abc-123"
Bad protocol version, id "abc-123", error -32602	/api/v4/orbit/mcp	"abc-123"	"abc-123"
Invalid jsonrpc value, id 7, error -32600	/api/v4/mcp	null	7
Invalid jsonrpc value, id 7, error -32600	/api/v4/orbit/mcp	null	7

The Orbit -32602 path already worked, because that endpoint stores the id in env inside the route block. The other three cases are fixed by this MR.

References

• Related issue: #611372 (closed)
• JSON-RPC 2.0 response object: https://www.jsonrpc.org/specification#response_object

Screenshots or screen recordings

Not applicable. This changes a JSON-RPC API response.

How to set up and validate locally

1. Enable the MCP server. In the Rails console run ApplicationSetting.current.update!(mcp_server_enabled: true).

2. Send a request with a bad protocol version and a known id. Confirm the error code is -32602 and the response id is "abc-123".

   curl --request POST --header "Authorization: Bearer $TOKEN" \
     --header "Content-Type: application/json" \
     --data '{"jsonrpc":"2.0","id":"abc-123","method":"initialize","params":{"protocolVersion":"1999-01-01"}}' \
     http://gdk.test:3000/api/v4/mcp

3. Repeat with "jsonrpc":"1.0" and id 7. Confirm the error code is -32600 and the response id is 7.

4. Run the specs.

   bundle exec rspec spec/requests/api/mcp/base_spec.rb \
     ee/spec/requests/api/orbit/mcp_spec.rb \
     ee/spec/requests/api/mcp/base_spec.rb

Specs ran locally: 109 examples, 0 failures. Steps 2 and 3 ran against both endpoints through the full request stack.

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