BREAKING(http/unstable): remove HEADER and METHOD

[tomas-zijdemans]

This removes the unstable HEADER and METHOD constants from @std/http. They mirror two IANA registries we'd have to track forever, and they add little over plain strings.

Rationale:

• They go stale. Both constants have drifted from the IANA registries since they landed in August 2024. fix(http/unstable): sync HEADER const with IANA registry #6991 and fix(http/unstable): sync METHOD with IANA registry #6992 exist only to resync them, and that work never ends: stabilizing these modules commits std to tracking the HTTP Field Name and Method registries indefinitely.
• The payoff is thin. The constants map names to the strings themselves (HEADER.ETag is "ETag"), and every web API involved (Headers, Request, fetch) takes plain strings. Typo protection is the main benefit, and editor autocomplete covers that.
• The cost isn't. Two modules, roughly 1,500 lines, for a list of string literals. file_server.ts was the only consumer in std, and the plain strings read fine there (see the diff).

Both modules have sat unstable for two years with no path to stabilization. Removing them now is cheap. Removing them after stabilization wouldn't be.

If we'd rather keep them, #6991 and #6992 should be merged instead so the constants at least match the current registries.

Earlier discussion: Discord thread

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.01%. Comparing base (df4d14d) to head (3410c8e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7239      +/-   ##
==========================================
- Coverage   95.03%   95.01%   -0.03%     
==========================================
  Files         619      616       -3     
  Lines       51499    51283     -216     
  Branches     9301     9300       -1     
==========================================
- Hits        48940    48724     -216     
  Misses       2021     2021              
  Partials      538      538              

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