fix(repo-server): expire cached Helm chart archives using repo cache expiration

[Mangaal]

This PR makes the repo server Helm chart cache respect the configured repo cache expiration.

Currently, downloaded Helm chart archives could remain in the on-disk chart cache even after the repo cache expires. This meant Argo CD could continue to extract a stale cached chart instead of refreshing it according to --repo-cache-expiration.

The change adds a chartCacheExpiration option to the Helm client and wires it from the repo server’s existing repo cache expiration setting. During chart extraction, the Helm client checks the cached chart archive’s modification time. If the archive is older than the configured expiration, it is removed and fetched again before extraction.

Closes #4002.

This PR doesn't fully address the issue where the Helm chart doesn't detect changes unless the version is updated. This can be used as a workaround for the root issue #24139.
Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
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[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds configurable Helm chart cache expiration and wires it through repo-server so cached chart archives are evicted after a TTL, with a regression test confirming expiration behavior.

Changes:

• Introduces WithChartCacheExpiration(time.Duration) and enforces expiration in nativeHelmChart.ExtractChart.
• Plumbs cache expiration from repo-server cache config into Helm client initialization.
• Adds a unit test + helper to validate cache eviction based on file mtime.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
util/helm/client.go	Adds chart-cache TTL option + eviction logic during chart extraction.
util/helm/client_test.go	Adds coverage ensuring cached charts are not reused past expiration.
reposerver/repository/repository.go	Wires chart-cache expiration into Helm client opts via init constants.
reposerver/cache/cache.go	Exposes repo cache expiration via a getter for wiring.
cmd/argocd-repo-server/commands/argocd_repo_server.go	Reads repo cache expiration and passes it into repo-server init constants.

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[codecov]

Codecov Report

❌ Patch coverage is 65.21739% with 8 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (master@19d99f5). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
util/helm/client.go	55.55%	6 Missing and 2 partials ⚠️

Additional details and impacted files

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  Coverage          ?   64.58%           
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  Files             ?      423           
  Lines             ?    58247           
  Branches          ?        0           
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  Hits              ?    37617           
  Misses            ?    17112           
  Partials          ?     3518           

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