Revert "CACHE-13504: remove gate from request cf"

[danlapid]

Reverts #6630

[ask-bonk]

This is a clean, exact revert of PR #6630. The diff in this PR (#6642) is the exact inverse of #6630's diff. The changes are:

1. Re-adding the workerdExperimental gate around the cacheControl synthesis and validation logic in http.c++
2. Re-adding --experimental flag to the test BUILD rule
3. Re-adding "experimental" compat flag to the .wd-test config
4. Re-adding the comment about requiring workerd_experimental in the test JS file

This is a straightforward, mechanically-generated revert. The code being restored was previously reviewed and merged. The revert itself is correct — every line matches the inverse of #6630. The PR has already been approved by two reviewers (mar-cf and jasnell).

LGTM

github run

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.49%. Comparing base (4495fba) to head (d973832).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/workerd/api/http.c++	90.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6642      +/-   ##
==========================================
- Coverage   66.50%   66.49%   -0.02%     
==========================================
  Files         405      405              
  Lines      117811   117814       +3     
  Branches    19428    19429       +1     
==========================================
- Hits        78348    78337      -11     
- Misses      27882    27898      +16     
+ Partials    11581    11579       -2     

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[codspeed-hq]

Merging this PR will degrade performance by 12.76%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 71 untouched benchmarks
⏩ 129 skipped benchmarks¹

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	Encode_ASCII_32[TextEncoder][0/0/32]	2.7 ms	3.1 ms	-12.76%

---

_{Comparing revert-6630-yawar/rm-exp-gate (d973832) with main (4495fba)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩