feat: Throw an error when WebAssembly is not available.

[ShogunPanda]

Summary

Report a recognizable, informative error when WebAssembly is unavailable for HTTP/1 parsing, while preserving APIs that do not require the parser. Client and dispatcher requests receive ERR_WEBASSEMBLY_NOT_SUPPORTED directly; fetch preserves it in error.cause.

Changes

• Define the error locally, independently of Node.js internal errors.
• Check WebAssembly availability during lazy parser initialization and defer Milo’s WASM loading.
• Preserve the SIMD → non-SIMD fallback.
• Add coverage for unavailable WASM, --jitless, Milo, error propagation, process usability, and WASM-independent APIs.
• Update TypeScript declarations and document the HTTP/1 requirement and fetch error cause.

Notes

Node.js integration requires a normal vendored Undici update. No alternative HTTP/1 parser is provided when WebAssembly is unavailable.

Assisted-By: OpenAI:GPT-6.1 Sol <openai/gpt-6.1-sol>
Related to nodejs/node#66533.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.50000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.92%. Comparing base (f850c68) to head (ba9ed8c).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/core/errors.js	76.47%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5966      +/-   ##
==========================================
+ Coverage   86.11%   86.92%   +0.80%     
==========================================
  Files         115      115              
  Lines       45686    45749      +63     
==========================================
+ Hits        39343    39767     +424     
+ Misses       6343     5982     -361     

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[mcollina]

lgtm