[PR #10113/01302134 backport][3.12] Restore 304 performance after fixing FileResponse replace race

[patchback]

This is a backport of PR #10113 as merged into master (0130213).

Followup to #10101 to address the feedback in #10112

We can avoid opening the file is we are going to send a 304.

fixes #10112

[codspeed-hq]

CodSpeed Performance Report

Merging #10118 will improve performances by 17.58%

_{Comparing patchback/backports/3.12/0130213423d8d055ece27c36a4dcc96e524cbc1c/pr-10113 (435b7c8) with 3.12 (cd0c2c8)}

Summary

⚡ 1 improvements
✅ 46 untouched benchmarks

Benchmarks breakdown

	Benchmark	3.12	patchback/backports/3.12/0130213423d8d055ece27c36a4dcc96e524cbc1c/pr-10113	Change
⚡	test_simple_web_file_response_not_modified[pyloop]	68 ms	57.9 ms	+17.58%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.06%. Comparing base (cd0c2c8) to head (435b7c8).
Report is 1 commits behind head on 3.12.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             3.12   #10118   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files         123      123           
  Lines       37137    37146    +9     
  Branches     4468     4467    -1     
=======================================
+ Hits        36419    36428    +9     
  Misses        540      540           
  Partials      178      178           

Flag	Coverage Δ
CI-GHA	97.95% <100.00%> (+<0.01%)	⬆️
OS-Linux	97.66% <100.00%> (-0.01%)	⬇️
OS-Windows	94.58% <100.00%> (+<0.01%)	⬆️
OS-macOS	96.78% <100.00%> (+<0.01%)	⬆️
Py-3.10.11	96.62% <100.00%> (-0.01%)	⬇️
Py-3.10.15	97.21% <100.00%> (+0.04%)	⬆️
Py-3.11.10	97.23% <100.00%> (-0.05%)	⬇️
Py-3.11.9	96.70% <100.00%> (+<0.01%)	⬆️
Py-3.12.7	97.72% <100.00%> (-0.01%)	⬇️
Py-3.13.0	96.56% <100.00%> (+<0.01%)	⬆️
Py-3.13.1	97.68% <100.00%> (-0.01%)	⬇️
Py-3.9.13	96.55% <100.00%> (+<0.01%)	⬆️
Py-3.9.20	97.13% <100.00%> (+0.04%)	⬆️
Py-pypy7.3.16	96.70% <100.00%> (+<0.01%)	⬆️
VM-macos	96.78% <100.00%> (+<0.01%)	⬆️
VM-ubuntu	97.66% <100.00%> (-0.01%)	⬇️
VM-windows	94.58% <100.00%> (+<0.01%)	⬆️

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