fix request signal

[Monti-27]

Fixes #5286.

This makes fetch(request) use the signal stored on the Request object.
It also keeps fetch level signal overrides working the same way.

Tests:

• cargo test -p boa_runtime --lib
• cargo clippy -p boa_runtime --all-features --all-targets -- -D warnings

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,735	50,735	0
Ignored	1,426	1,426	0
Failed	802	802	0
Panics	0	0	0
Conformance	95.79%	95.79%	0.00%

Tested main commit: f5e88de558e038f0ae675a012d59917a098f44b6
Tested PR commit: 96081c42d51d8d5927c8fa95891085a0635a2956
Compare commits: f5e88de...96081c4

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.72%. Comparing base (6ddc2b4) to head (96081c4).
⚠️ Report is 946 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5288       +/-   ##
===========================================
+ Coverage   47.24%   59.72%   +12.47%     
===========================================
  Files         476      589      +113     
  Lines       46892    63490    +16598     
===========================================
+ Hits        22154    37917    +15763     
- Misses      24738    25573      +835     

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[HalidOdat]

Looks good to me! 😄