fix(jsonrpc2): decode requests when method key is present

[piyushbag]

Summary

Fixes #976.

DecodeMessage used msg.Method != "" to classify messages. A wire payload with "method":"" was decoded as a response, so stdio servers dropped it without a correlated JSON-RPC error.

Approach

Decode into a wireDecode struct whose method field is json.RawMessage. Treat the message as a request when the method key is present on the wire (including ""), which matches JSON-RPC semantics.

This revision:

• Uses a single JSON parse per message (dedicated decode struct, no secondary fields-map unmarshal)
• Documents why json.RawMessage is used for method
• Adds regression coverage so normal responses and id-only payloads without a method key stay on the response path

Test plan

• go test ./internal/jsonrpc2/... -v
• go test ./mcp/... -run TestIOConnRead_EmptyMethod -v
• go test ./... -count=1
• go test -race ./internal/jsonrpc2/... -count=1
• Server conformance (npx @modelcontextprotocol/conformance server --url http://127.0.0.1:3000/mcp) — 40 passed, 0 failed
• Client conformance (./scripts/client-conformance.sh) — 217 passed, 0 failed

Scope note

This PR addresses the primary repro in #976 (empty method with valid id). Other cases listed in the issue (id: null, structurally invalid envelopes) are unchanged.