internal/jsonrpc2: allow EOF-side responses to drain

[ychampion]

Read-side EOF currently marks the whole connection as shutting down before handlers for already-read calls can write their responses. For stdio servers driven by one-shot pipes, that drops responses even though stdout is still writable.

This treats io.EOF as a read half-close for in-flight responses: EOF no longer cancels already accepted incoming requests, and response writes only reject explicit close, broken writer, or non-EOF read errors.

Fixes #1061

Validation:

• red-before: go test ./internal/jsonrpc2 -run TestReadEOFAllowsInFlightResponseToDrain -count=1 failed with Wait returned error: server is closing: EOF
• go test ./internal/jsonrpc2 -run TestReadEOFAllowsInFlightResponseToDrain -count=1
• go test ./internal/jsonrpc2 -count=1
• go test ./mcp -run 'Test(Stdio|Server|Client|InMemory)' -count=1\n- go test ./...\n- go vet ./...\n- git diff --check