feat(http/unstable): accept HeadersInit for serveDir and serveFile headers

[minato32]

Closes #6723.

ServeDirOptions.headers and ServeFileOptions.headers in @std/http/unstable-file-server now accept HeadersInit instead of string[], so callers can pass a Headers instance, a record, or a list of tuples without hand-rolling name:value strings.

The stable serveDir/serveFile keep their existing string[] signature, so the file-server CLI behaviour is unchanged. The unstable wrapper strips headers before delegating to the stable implementation and applies them to the response itself (skipping redirect responses, matching the stable behaviour).

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[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.63%. Comparing base (f0431c2) to head (6f87cb1).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
http/unstable_file_server.ts	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7190      +/-   ##
==========================================
- Coverage   94.64%   94.63%   -0.02%     
==========================================
  Files         629      630       +1     
  Lines       51895    51902       +7     
  Branches     9373     9376       +3     
==========================================
- Hits        49116    49115       -1     
- Misses       2211     2217       +6     
- Partials      568      570       +2     

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[bartlomieju]

Nice ergonomic improvement — accepting HeadersInit is much friendlier than hand-built "name:value" strings, and the implementation reads cleanly. Verified the redirect-skip matches stable serveDir and that headers is correctly stripped before delegating so there's no double-application. Coverage is good (record + Headers instance + redirect skip, for both serveDir/serveFile).

A couple of non-blocking notes inline. Also worth calling out in the changelog: this is a hard break for the unstable API (old headers: ["X-Extra: extra header"] callers now fail to type-check and throw at runtime via new Headers(...)). That's fine for unstable — just be explicit about it. LGTM overall.

[bartlomieju]

Using .append() means custom headers are added alongside any the file server already set, rather than replacing them — e.g. passing cache-control would yield a comma-joined value rather than overriding. This matches the previous string[] behavior so it's not a regression, but now that the API takes a rich HeadersInit, callers may reasonably expect to override (e.g. set their own Cache-Control). Worth a one-line doc note on the headers option clarifying that values are appended, not replaced. (The cache-control test only passes because the dir-listing response doesn't pre-set that header.)

[bartlomieju]

The as UnstableServeDirOptions cast (repeated at 5 call sites) is forced by Omit<StableServeDirOptions, "headers"> + redefine: the spread carries stable's headers?: string[], which isn't assignable to HeadersInit. You could avoid all the casts by typing the shared serveDirOptions fixture as UnstableServeDirOptions (or using satisfies). Minor, test-only.

[minato32]

@bartlomieju updated in 6f87cb1 — added a doc note on both headers options clarifying values are appended (not replaced), and switched the test fixture to satisfies ServeDirOptions so the per-call casts are gone. Thanks for the review!