feat(runtime): add Request.clone() and Response.clone() per Fetch Standard §7.3/§7.4

[Nakshatra480]

Summary

Closes #4992

Implements clone() on Request and Response objects in boa_runtime per Fetch Standard §7.3 and §7.4.

Changes

• headers.rs: Added pub(crate) fn deep_clone() on JsHeaders to produce a fully independent header map copy.
• request.rs: Added body_used: Cell<bool> tracking field and clone() method. Throws TypeError if body is already used.
• response.rs: Added body_used: Cell<bool> tracking field and clone() method. Throws TypeError if body is already used. bytes(), text(), and json() now also throw TypeError on a consumed body and mark it as used.

Behavior

• request.clone() returns a new Request with an independent copy of method, headers, and body. Shares AbortSignal when signal support is wired into JsRequest (follow-up).
• response.clone() returns a new Response with independent copies of status, headers, body, and url.
• Both throw TypeError if the body has already been consumed.

Testing

• cargo check -p boa_runtime passes
• cargo clippy --all-features --all-targets -- -D warnings passes
• cargo fmt --all -- --check passes
• cargo test -p boa_runtime passes

Spec References

• https://fetch.spec.whatwg.org/#dom-request-clone
• https://fetch.spec.whatwg.org/#dom-response-clone

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,073	50,073	0
Ignored	2,072	2,072	0
Failed	818	818	0
Panics	0	0	0
Conformance	94.54%	94.54%	0.00%

Tested main commit: daf86d30277a5c1fb353825347f75c1d83516905
Tested PR commit: 0a0960383ec9d20a8db1b0c8ea01d89250733fbf
Compare commits: daf86d3...0a09603

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.36%. Comparing base (6ddc2b4) to head (0a09603).
⚠️ Report is 885 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4993       +/-   ##
===========================================
+ Coverage   47.24%   59.36%   +12.11%     
===========================================
  Files         476      580      +104     
  Lines       46892    63160    +16268     
===========================================
+ Hits        22154    37493    +15339     
- Misses      24738    25667      +929     

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[Nakshatra480]

Hi @jedel1043 can you pls review this pr whenever you get a chance.

[jedel1043]

LGTM

Ping me when you resolve the conflicts so that we can merge

[Nakshatra480]

@jedel1043 i have resolved the merge conflicts.

[Nakshatra480]

Thanks Jedel