fix(agent): preserve pools with pending GOAWAY replays

[mcollina]

This relates to...

Spurious ClientDestroyedError disconnects can occur while using fetch() with AbortSignal.timeout() against an HTTP/2 destination that sends GOAWAY.

Rationale

When GOAWAY detaches an HTTP/2 session, refused requests are moved back to the Client's pending queue before a replacement connection is opened. During that window the owning Agent sees no connected or busy dispatcher and closes the Pool, despite the pending replays. Once those requests finish on the replacement Client, the closing Pool destroys it and emits an unexpected UND_ERR_DESTROYED disconnect.

Before this change the regression produced disconnect codes:

UND_ERR_INFO
UND_ERR_DESTROYED

The second disconnect is internal cleanup rather than a request failure.

Changes

Features

N/A

Bug Fixes

• Keep an Agent-managed dispatcher alive while it has pending requests.
• Add HTTP/2 regression coverage using concurrent fetch() requests, AbortSignal.timeout(3000), and a server-issued GOAWAY.
• Assert that requeued requests complete and only the expected GOAWAY informational disconnect is emitted.

Breaking Changes and Deprecations

N/A

Status

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

Validation

• npm run test:h2:core — 104 passed, 2 skipped
• npx borp --timeout 180000 -p "test/node-test/agent.js" — 35 passed
• npx borp --timeout 180000 -p "test/issue-4806.js" — passed
• npx eslint lib/dispatcher/agent.js test/http2-goaway.js — passed

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.46%. Comparing base (fc3450d) to head (a7652e7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5740      +/-   ##
==========================================
- Coverage   93.47%   93.46%   -0.01%     
==========================================
  Files         110      110              
  Lines       38908    38913       +5     
==========================================
+ Hits        36368    36371       +3     
- Misses       2540     2542       +2     

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