Fix flaky Playground local cache tests

[bvolpato]

Summary

Two local-cache tests depended on runner timing or unsynchronized state:

• TestLocalCache_GetValue and TestLocalCache_SetValue expired fixtures one millisecond after setup. Slow runners could cross that deadline before assertions ran. Their non-expired fixtures now use a one-minute deadline.
• TestLocalCache_startGC slept for one millisecond, then read the cache map while the GC goroutine could delete from it. It now polls for cleanup within a bounded deadline while holding the cache mutex. Each subtest owns its context and expiration state, and the nil-cache case still verifies that GC stops.

Production cache behavior is unchanged.

Reproduction

The scheduled Playground precommit failed in TestLocalCache_SetValue/Set_value because GetValue reported the newly stored value as missing:

https://github.com/apache/beam/actions/runs/33273173203

The GC race reproduced immediately with:

go test -race ./internal/cache/local -run '^TestLocalCache_startGC$'

Race detector reported an unlocked test read at local_cache_test.go:575 concurrent with deletion in Cache.clearItems.

Testing

• cd playground/backend && go test ./internal/cache/local
• cd playground/backend && go test ./internal/cache/local -run '^TestLocalCache_(GetValue|SetValue)$' -count=10000
• cd playground/backend && go test -race ./internal/cache/local -run '^TestLocalCache_(GetValue|SetValue)$' -count=100
• cd playground/backend && go test -race ./internal/cache/local -run '^TestLocalCache_startGC$' -count=1000
• cd playground/backend && go test -race ./internal/cache/local -count=1

./gradlew :playground:backend:test also ran the changed cache package successfully, then stopped on unrelated local emulator and Scio setup failures.

---

• No tracking issue; failing CI run and race reproducer are included above.
• CHANGES.md is unchanged because this is a test-only stabilization.
• This contribution is not large.

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