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 90.90909% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.38%. Comparing base (f9dd97e) to head (a95113d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/http/handler/hijack.go	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16625      +/-   ##
==========================================
+ Coverage   80.28%   80.38%   +0.09%     
==========================================
  Files         217      217              
  Lines       13568    13589      +21     
==========================================
+ Hits        10893    10923      +30     
+ Misses       2312     2303       -9     
  Partials      363      363              

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[dprotaso]

I don't know if we want to introduce websocket hijacking into the queue proxy. The idea before was to track this transparently so we're not pulling in extra dependencies etc.