fix(h2): unref a session detached by GOAWAY with no open streams

[kjsik11]

This relates to...

Regression from #5839

resumeH2 (#5839): keeps the session ref'd while requests are queued.
onHttp2SessionGoAway (this PR): unrefs the session detached by GOAWAY when it has no open streams.

Rationale

When the peer advertises SETTINGS_MAX_CONCURRENT_STREAMS = 0, requests are queued without a stream.
After the peer sends GOAWAY, the session is detached but never unref'd, so it keeps the process alive.

Changes

• lib/dispatcher/client-h2.js: onHttp2SessionGoAway() unrefs the detached session if it has no open streams
• test/http2-max-concurrent-streams-zero-unref.js: the peer sends GOAWAY while a request is queued. The child must exit after the request completes

Bug Fixes

• A session detached by GOAWAY with no open streams is now unref'd

Tests (Node 24.20.0)

• npm run lint
• npm run test:h2:core (122/122)
• npx borp -p "test/http2-max-concurrent-streams-zero-unref.js" (3/3,main 2/3)

Assisted-by: Claude Code

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• [S] Benchmarked (optional)
• [S] Documented
• Review ready
• In review
• Merge ready

[mcollina]

lgtm