fix(http/unstable): use relative import for unstable_structured_fields

[bartlomieju]

The 2026.05.26 release publish failed for @std/http@1.1.1 with
export 'unstable-structured-fields' not found in jsr:@std/http.
The cause is a self-import in unstable_message_signatures.ts: it
pulled types and helpers from @std/http/unstable-structured-fields
rather than from the sibling file. At publish time JSR resolves that
specifier against the previously published @std/http (1.1.0), which
does not yet expose the new unstable-structured-fields export — both
modules were added in this release cycle — so the module graph build
errors out before the new version can be published.

Switching the two import sites to a relative
./unstable_structured_fields.ts path avoids the registry lookup and
unblocks the release. No other real imports in the http package use the
@std/http/* self-specifier; the remaining occurrences are all inside
JSDoc examples and do not enter the module graph.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (45ff477) to head (fa34094).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7155   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files         636      636           
  Lines       52138    52138           
  Branches     9399     9399           
=======================================
  Hits        49311    49311           
  Misses       2249     2249           
  Partials      578      578           

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