feat: support additional RequestInit fields

[sahanaxzy]

Summary

Extends RequestInit with standard Fetch options (mode, credentials, cache, redirect, referrer, referrerPolicy, integrity, keepalive), bringing new Request(url, init) closer to browser behavior.

Closes #4987

Changes

Modified fetch/request.rs

• Added the missing RequestInit fields listed above
• Added validation for mode, credentials, cache, redirect, and referrerPolicy
• referrer, integrity, and keepalive are accepted but currently no-ops (ready to wire up later)

Modified fetch/tests/request.rs

• Added tests for valid mode/credentials acceptance and invalid mode rejection

Testing

cargo fmt --all
cargo test -p boa_runtime --lib
cargo clippy -p boa_runtime --all-features --all-targets

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,935	49,935	0
Ignored	2,207	2,207	0
Failed	821	821	0
Panics	0	0	0
Conformance	94.28%	94.28%	0.00%

Tested main commit: cffb50c2c37cf7905911ba6446667d8e182d29ad
Tested PR commit: 7fe2c747949959b96273133f6e35af7ed0b02e33
Compare commits: cffb50c...7fe2c74

[codecov]

Codecov Report

❌ Patch coverage is 38.09524% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.01%. Comparing base (6ddc2b4) to head (7fe2c74).
⚠️ Report is 860 commits behind head on main.

Files with missing lines	Patch %	Lines
core/runtime/src/fetch/request.rs	38.09%	39 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4990       +/-   ##
===========================================
+ Coverage   47.24%   59.01%   +11.76%     
===========================================
  Files         476      563       +87     
  Lines       46892    62621    +15729     
===========================================
+ Hits        22154    36954    +14800     
- Misses      24738    25667      +929     

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[sahanaxzy]

@jedel1043 can you review this...

[jedel1043]

Needs rebase