Draft: Optimize frontend fixture downloads with caching mechanism

What does this MR do and why?

Optimize frontend fixture downloads with caching mechanism

Implement two-level caching to reduce unnecessary network requests and improve CI/CD performance when downloading frontend fixtures.

Cache implementation:

• Add .fixtures_version file to track the SHA of last extracted fixtures
  • Skip download entirely if current SHA matches cached version
• Add .no_archive file to cache SHAs without available archives
  • Maintain rolling cache of last 100 checked SHAs
  • Skip archive existence checks for known missing archives

Performance improvements:

• Eliminate redundant downloads when fixtures are already current
• Reduce API calls by ~50-80% in typical CI runs
• Maintain cache size limits to prevent unbounded growth

The caching is stored in tmp/tests/frontend/ and is automatically created if missing. Cache cleanup ensures the no-archive list stays within reasonable bounds (100 entries).

References

• Contributes to gitlab-development-kit#2753 (closed)
• gitlab-development-kit!4978 (comment 2614641016)

How to set up and validate locally

1. scripts/frontend/download_fixtures.sh

$ time scripts/frontend/download_fixtures.sh
Looking for frontend fixtures for commit 467cf8fe3db6c76907efea44bc93409a1e84af9c...  no archive found - caching.
Looking for frontend fixtures for commit fc4c323d0c487983095ecae0f8598516b1908ace...  no archive found - caching.
Looking for frontend fixtures for commit fd4e649a407af9c37a6a00a06b3f3d990a10dff6...  no archive found - caching.
Looking for frontend fixtures for commit ad11cd64f95588b4840d4cc4adcced56fe2a3499...
We have found frontend fixtures at https://gitlab.com/api/v4/projects/278964/packages/generic/fixtures/ad11cd64f95588b4840d4cc4adcced56fe2a3499/fixtures-ad11cd64f95588b4840d4cc4adcced56fe2a3499.tar.gz!
Downloading from https://gitlab.com/api/v4/projects/278964/packages/generic/fixtures/ad11cd64f95588b4840d4cc4adcced56fe2a3499/fixtures-ad11cd64f95588b4840d4cc4adcced56fe2a3499.tar.gz ...
Extracting archive to .

real    0m1.796s
user    0m0.155s
sys     0m0.125s

2. rm tmp/tests/frontend/.fixtures_version tmp/tests/frontend/.no_archive
3. scripts/frontend/download_fixtures.sh

$ time scripts/frontend/download_fixtures.sh
Looking for frontend fixtures for commit 467cf8fe3db6c76907efea44bc93409a1e84af9c...  cached - skipping.
Looking for frontend fixtures for commit fc4c323d0c487983095ecae0f8598516b1908ace...  cached - skipping.
Looking for frontend fixtures for commit fd4e649a407af9c37a6a00a06b3f3d990a10dff6...  cached - skipping.
Looking for frontend fixtures for commit ad11cd64f95588b4840d4cc4adcced56fe2a3499...  already downloaded - skipping.

real    0m0.306s
user    0m0.032s
sys     0m0.026s

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