stacklok / toolhive / 24497409662

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Expand singleflight boundary in ValidatingCache.Get to cover full hit+check and miss+load path (#4798)

Move the singleflight.Do boundary to wrap the entire Get operation rather than
only the miss+load path. This coalesces concurrent liveness checks for the same
key into a single storage.Load round-trip, reducing storage pressure under
concurrent access.

With only one goroutine running per key at a time:
- ContainsOrAdd is replaced with plain Add — the concurrent-writer race between
  multiple Get goroutines no longer exists
- The ContainsOrAdd + Get race-handling block is removed entirely
- The onEvict field on the struct is removed (it was only needed to manually
  call the callback in the discarded-load path)

As a side effect of the new design, an expired cache hit now evicts the entry
and falls through to load within the same singleflight operation, returning the
fresh value instead of (zero, false).

Closes #4729

Co-authored-by: taskbot <taskbot@users.noreply.github.com>

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