fix: decode empty jsonrpc methods as requests

[he-yufeng]

Fixes #976.

An incoming JSON-RPC message with an explicit empty method field was decoded as a response because the wire struct could not distinguish a missing method from an empty method. That meant stdio servers silently ignored requests like id=5/method="" instead of routing them through request handling and returning a correlated JSON-RPC error.

This keeps method presence during decode by storing the wire method as a pointer, then decodes method="" as a Request. The existing dispatch path can then return method-not-found for the request id while leaving the session alive.

Validation:

• go test ./internal/jsonrpc2
• go test ./mcp -run "TestIOConnRead|TestServerSessionHandle|TestKeepAliveMethodNotFound"
• go test ./...