test: preserve Content-Type in stubFetch response clone on Deno 2.x

[zimeg]

Summary

This pull request fixes a test (v2.x) failure in the Deno CI matrix caused by a Response.clone() behavior change on Deno 2.x.

• On Deno 2.x, Response.clone() drops headers set from a plain object literal (e.g. { "Content-Type": "application/json" }). Headers set via a Headers instance survive.
• stubFetch returned response.clone() directly, so a stubbed response built with plain-object headers lost its Content-Type — breaking testing/http_test.ts on v2.x (response.headers.get("Content-Type") returned null).
• Fix: reconstruct the response from a fresh new Headers(response.headers) copy so the stub preserves headers regardless of how the caller built the Response.
• Also set fail-fast: false on the Deno test matrix — the v2.x failure was cancelling the v1.x and v1.46.2 jobs, hiding that those versions actually passed.

testing

• Check out this branch and run deno task test — the full suite (fmt, lint, 37 tests) passes on Deno 2.x, where stubFetch's "should replace global fetch with a stub" previously failed.
• Confirm the Deno CI matrix now reports each version (v1.x, v1.46.2, v2.x) independently rather than cancelling siblings on first failure.

Special notes

Root cause isolated with a minimal repro: a Response built with headers: { "Content-Type": "application/json" } returns null for that header after .clone() on Deno 2.x, while the same built with new Headers({...}) preserves it.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've ran deno task test after making the changes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.96%. Comparing base (a5e5839) to head (4b397eb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #159   +/-   ##
=======================================
  Coverage   96.95%   96.96%           
=======================================
  Files          39       39           
  Lines        1643     1648    +5     
  Branches       25       25           
=======================================
+ Hits         1593     1598    +5     
  Misses         47       47           
  Partials        3        3           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[zimeg]

🗣️ Unblocks #157 and #158

[WilliamBergamin]

Nice fix 💯