fix: wait for hijacked connections to close during drain

[immanuwell]

Fixes #

Proposed Changes

• keep hijacked conns counted til the wrapped net.Conn closes
• preserve Flush() and Unwrap() on the tracker wrapper
• add regression tests for a real Hijack() path, not just a blocked handler

Repro:

1. Open a websocket to a Knative Service.
2. Send TERM to the queue-proxy while that socket is still open.
3. Before this patch, the handler can return right after Hijack(), so HijackedDrainer.Drain() may see zero inflight too early and exit. The websocket gets cut off early, which is kinda rough.
4. With this patch, drain waits for the hijacked conn to close, or hits the existing 60s cap.

Related: follow-up to #16362.

Release Note

queue-proxy now waits for hijacked connections to close before finishing drain

[knative-prow]

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[knative-prow]

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[codecov]

Codecov Report

❌ Patch coverage is 84.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.34%. Comparing base (516bc43) to head (751dd09).

Files with missing lines	Patch %	Lines
pkg/http/handler/hijack.go	84.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16625      +/-   ##
==========================================
- Coverage   80.36%   80.34%   -0.03%     
==========================================
  Files         217      217              
  Lines       13568    13592      +24     
==========================================
+ Hits        10904    10920      +16     
- Misses       2301     2307       +6     
- Partials      363      365       +2     

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[dprotaso]

Before this patch, the handler can return right after Hijack(), so HijackedDrainer.Drain() may see zero inflight too early and exit.

Are any of our handlers in serving returning after hijack? I would be surprised if they were - that's why this change seems unnecessary to me