Fix WebSocketResponse.prepared not correctly reflect the WebSocket's prepared state

[bdraco]

This PR fixes an issue where the WebSocketResponse.prepared property did not correctly reflect the WebSocket's prepared state, particularly during timeout scenarios.

Previously, the prepared property inherited from StreamResponse only checked for _payload_writer, but in WebSocket timeout scenarios, the _writer could be set while _payload_writer remained None, leading to incorrect state reporting.

Changes

• Added an explicit prepared property to WebSocketResponse that checks for _writer instead of relying on the parent class's _payload_writer check
• Added a check in the prepare() method to prevent double preparation when _writer is already set
• Added comprehensive tests covering timeout scenarios and edge cases where WebSocket state needs to be correctly tracked

fixes #6009

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.77%. Comparing base (1878509) to head (2cb4df5).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10971      +/-   ##
==========================================
+ Coverage   98.10%   98.77%   +0.66%     
==========================================
  Files         129      129              
  Lines       39541    39620      +79     
  Branches     2185     2186       +1     
==========================================
+ Hits        38792    39133     +341     
+ Misses        591      339     -252     
+ Partials      158      148      -10     

Flag	Coverage Δ
CI-GHA	98.65% <100.00%> (+0.65%)	⬆️
OS-Linux	98.36% <100.00%> (+0.66%)	⬆️
OS-Windows	96.54% <100.00%> (+0.54%)	⬆️
OS-macOS	97.51% <100.00%> (-0.01%)	⬇️
Py-3.10.11	97.40% <100.00%> (+0.29%)	⬆️
Py-3.10.17	97.93% <100.00%> (+0.63%)	⬆️
Py-3.11.12	98.02% <100.00%> (?)
Py-3.11.9	97.48% <100.00%> (+0.29%)	⬆️
Py-3.12.10	98.42% <100.00%> (+0.93%)	⬆️
Py-3.13.3	98.40% <100.00%> (+1.10%)	⬆️
Py-3.9.13	97.28% <100.00%> (+<0.01%)	⬆️
Py-3.9.22	97.80% <100.00%> (+0.66%)	⬆️
Py-pypy7.3.16	84.87% <9.23%> (?)
VM-macos	97.51% <100.00%> (-0.01%)	⬇️
VM-ubuntu	98.36% <100.00%> (+0.66%)	⬆️
VM-windows	96.54% <100.00%> (+0.54%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #10971 will not alter performance

_{Comparing fix_6009 (2cb4df5) with master (48f5324)}

Summary

✅ 59 untouched benchmarks

[patchback]

Backport to 3.11: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.11/1ee187c08c9bb59acb53e1c80a06c79a4183de10/pr-10971

Backported as #10982

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.12: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.12/1ee187c08c9bb59acb53e1c80a06c79a4183de10/pr-10971

Backported as #10983

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.