feat(http): stabilize Problem Details

[tomas-zijdemans]

Stabilizes @std/http/problem-details, the RFC 9457 Problem Details module. It has been unstable since #7033 (March 2026), was reshaped once in #7117, and has sat untouched since: no open issues, no bug reports, 38 tests.

Changes

• Renames http/unstable_problem_details.ts to http/problem_details.ts (test file too) and moves the export to @std/http/problem-details
• Removes the @experimental tags
• Re-exports from http/mod.ts and adds a short Problem Details section to the package doc
• Rewords the isProblemDetailsResponse doc: it claimed "type guard" but the function returns a plain boolean

Two behaviors that freeze at 1.x

Both are deliberate and documented. Flagging them here so they get a look now rather than in an issue later:

1. parseProblemDetails throws TypeError on a Response. A Response is a non-null, non-array object with no enumerable own keys, so it would otherwise parse silently to an empty object. The error points callers at parseProblemDetailsResponse.
2. parseProblemDetailsResponse parses any JSON body without checking the content type. Detection belongs to isProblemDetailsResponse, so the pair composes without checking twice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (cdb83c6) to head (b1af76f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7270      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         618      617       -1     
  Lines       51596    51593       -3     
  Branches     9340     9340              
==========================================
- Hits        49035    49031       -4     
  Misses       2021     2021              
- Partials      540      541       +1     

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