## 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
- [x] `go test ./internal/jsonrpc2/... -v`
- [x] `go test ./mcp/... -run TestIOConnRead_EmptyMethod -v`
- [x] `go test ./... -count=1`
- [x] `go test -race ./internal/jsonrpc2/... -count=1`
- [x] Server conformance (`npx @modelcontextprotocol/conformance server
--url http://127.0.0.1:3000/mcp`) — 40 passed, 0 failed
- [x] 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.
Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
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