fix: preserve empty-method requests during decode

[he-yufeng]

Summary

Fixes #976.

DecodeMessage used msg.Method != "" to distinguish requests from responses. That means a wire payload with a present but empty method field, such as:

{"jsonrpc":"2.0","id":5,"method":"","params":{}}

was decoded as a response instead of a request. Since there was no matching outgoing call, the server ignored it and did not send a correlated JSON-RPC error, even though the session stayed alive.

This PR checks whether the wire payload contains the method field instead of checking whether the decoded string is non-empty. An empty method now reaches the normal request dispatch path and receives the existing method-not-found handling.

Tests

go test ./internal/jsonrpc2
go test ./mcp
go test ./...
git diff --check