fix(asynccache): pass cached values to delete handlers

[Shubham-Padkonde]

DeleteHandler currently receives the cache's private *entry wrapper. A handler expecting the stored value gets the wrong type, both when an entry expires and when DeleteIf removes it.

Load the cached value before dispatching either asynchronous callback, and document the argument. Add six regression cases for strings, pointers, and nil values across both deletion paths. The tests also check that non-matching/touched entries remain and the first expiration pass does not delete an entry.

Fixes #248.

Validation:

• All six regression cases fail before the change and pass afterward.
• Full repository go test -v -race -covermode=atomic ./... passes with Go 1.15.15, matching the CI Go series.
• go vet -stdmethods=false ./... and repository-wide gofmt -s checks pass with Go 1.15.15.
• Focused regression race checks also pass with Go 1.26.3.
• Golangci-lint 2.10.1 reports no findings in changed code (--new-from-rev=HEAD). Its full-repository defaults report 52 findings on unchanged code; these are not included in this callback fix.
• git diff --check passes.

Assisted by Codex/GPT-6 for investigation, coding, and validation.

[PureWhiteWu]

LGTM, thanks very much for the fix!

[PureWhiteWu]

Please change your target branch to main

The base branch was changed.

[Shubham-Padkonde]

@PureWhiteWu Retargeted to main and updated the branch with the latest main. The diff is still just the two asynccache files. The base change auto-dismissed your earlier approval, so could you please re-approve? The required build check hasn't started yet, so if it needs maintainer approval to run, could you approve that too? Thanks!