response_writer: don't panic on Hijack/CloseNotify when wrapper unsupported

[SAY-5]

Closes #4638. http.TimeoutHandler's timeoutWriter doesn't implement http.Hijacker/http.CloseNotifier — direct type assertion panics. Match the existing Flush() graceful-degradation pattern (#4479): return http.ErrNotSupported from Hijack and nil from CloseNotify.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (3dc1cd6) to head (08539d9).
⚠️ Report is 275 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4645      +/-   ##
==========================================
- Coverage   99.21%   98.37%   -0.84%     
==========================================
  Files          42       48       +6     
  Lines        3182     3143      -39     
==========================================
- Hits         3157     3092      -65     
- Misses         17       42      +25     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.36% <100.00%> (?)
-tags go_json	98.30% <100.00%> (?)
-tags nomsgpack	98.35% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.25	98.37% <100.00%> (?)
go-1.26	98.37% <100.00%> (?)
macos-latest	98.37% <100.00%> (-0.84%)	⬇️
ubuntu-latest	98.37% <100.00%> (-0.84%)	⬇️

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[SAY-5]

The only failing check is codecov/project (project-level coverage delta). codecov/patch itself passes. The patch is two early-return guards in response_writer.go for unsupported writer types; happy to add a short test if helpful but the check should not block.