fix(fetch): handle BufferSource bodies in Response

[iammdzaidalam]

Closes #5216

Adds BufferSource handling to the Response() constructor body extraction path.

Currently, inputs like Uint8Array or DataView fall through to stringification,
so binary bodies end up as strings (e.g. "[object Uint8Array]") instead of raw bytes.

This change:

• handles ArrayBuffer, typed arrays, and DataView
• slices typed array / DataView inputs using byteOffset / byteLength
• uses usize::try_from(...) for offsets/lengths to avoid truncation on 32-bit targets
• preserves the existing string fallback for non-binary inputs
• leaves other BodyInit types (e.g. Blob, FormData) for follow-up

Testing

Added tests in core/runtime/src/fetch/tests/response.rs covering:

• ArrayBuffer
• Uint8Array
• subarray/view slicing
• DataView with non-zero offset/length
• string fallback behavior
• content-type behavior (binary vs string)

Notes

• binary bodies do not inject a default Content-Type
• string bodies still default to text/plain;charset=UTF-8

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,578	53,578	0
Passed	51,435	51,435	0
Ignored	1,622	1,622	0
Failed	521	521	0
Panics	0	0	0
Conformance	96.00%	96.00%	0.00%

Tested main commit: 8a506c8831c6bd76b5640275821708d0f3a9b0a6
Tested PR commit: 0b98482a7a443ada21e35194d5cb30f41470118a
Compare commits: 8a506c8...0b98482

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.98%. Comparing base (6ddc2b4) to head (0b98482).
⚠️ Report is 1047 commits behind head on main.

Files with missing lines	Patch %	Lines
core/runtime/src/fetch/response.rs	92.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5217       +/-   ##
===========================================
+ Coverage   47.24%   62.98%   +15.74%     
===========================================
  Files         476      535       +59     
  Lines       46892    60080    +13188     
===========================================
+ Hits        22154    37844    +15690     
+ Misses      24738    22236     -2502     

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[iammdzaidalam]

Hey @jedel1043, any updates here when you get a moment?

[iammdzaidalam]

hey @jedel1043 @nekevss ,any updates when u get a moment?

[jedel1043]

Looks great! It just needed an update against main, but we can merge now.