log undeliverable late responses in processResult

[o-mid]

Summary

• Report processResult response write failures through OnInternalError instead of dropping them.
• Add a unit test that a failed response write invokes the hook with the wrapped write error.

Fixes #1147.

Motivation

When a handler finishes after the client has already torn down the stream (e.g. client-side timeout), c.write fails and that error was discarded. Server logs then look like nothing happened. Maintainer guidance on the issue was to log at that site; MCP already wires OnInternalError to slog in mcp/transport.go.

Test plan

• go test ./internal/jsonrpc2/ -run TestProcessResultWriteFailureReportsInternalError
• go test ./internal/jsonrpc2/...
• CI green