[Prism] Fix race condition in element manager

[shunping]

Ensures 'refreshCond.Broadcast()' is fully synchronized with its mutex lock 'refreshCond.L' during pipeline termination and processing-time timer scheduling ('wakeUpAt').

This avoids race conditions causing lost wake-up signals and indefinite test hangs.

Without this change, we have seen a rare deadline exceed on random tests. The goroutine dump suggests that it is stuck in the watermark evaluation loop while the pending element goroutine has ended.

2026-05-28T13:06:16.6677165Z 1 @ 0x48ddee 0x48f833 0x48f813 0x49e9f3 0xc0fc25 0x495621
2026-05-28T13:06:16.6678046Z #  0x48f812        sync.runtime_notifyListWait+0x192                                                                               /usr/local/go/src/runtime/sema.go:617
2026-05-28T13:06:16.6679030Z #  0x49e9f2        sync.(*Cond).Wait+0x72                                                                                          /usr/local/go/src/sync/cond.go:71
2026-05-28T13:06:16.6680753Z #  0xc0fc24        [github.com/apache/beam/sdks/v2/go/pkg/beam/runners/prism/internal/engine.(*ElementManager).Bundles.func2+0x244](https://www.google.com/url?sa=D&q=http%3A%2F%2Fgithub.com%2Fapache%2Fbeam%2Fsdks%2Fv2%2Fgo%2Fpkg%2Fbeam%2Frunners%2Fprism%2Finternal%2Fengine.(*ElementManager).Bundles.func2%2B0x244)  /runner/_work/beam/beam/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go:426
2026-05-28T13:06:16.6682188Z

Example:
https://github.com/apache/beam/actions/runs/26588248130/job/78339963103?pr=38713

[shunping]

r: @lostluck

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a race condition in the Prism runner's ElementManager that occasionally caused pipeline termination to hang. By ensuring that condition variable broadcasts are properly synchronized with their associated mutex locks, the change guarantees that wake-up signals are correctly received by the watermark evaluation loop. Additionally, minor adjustments were made to test infrastructure to improve CI stability.

Highlights

• Race Condition Resolution: Wrapped 'refreshCond.Broadcast()' calls with 'refreshCond.L.Lock()' and 'Unlock()' in the ElementManager to ensure proper synchronization and prevent lost wake-up signals.
• Test Stability: Updated test configuration files and Gradle tasks to improve reliability and prevent sporadic test hangs in the Prism runner.

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[github-actions]

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[gemini-code-assist]

Code Review

This pull request triggers a Python post-commit test suite, configures the Prism Java ValidatesRunner task to always run by disabling up-to-date caching, and fixes potential lost wake-up signals in the Go SDK's element manager by locking the mutex before calling Broadcast on refreshCond. A review comment suggests simplifying the Go code by removing an unnecessary outer goroutine wrapper around time.AfterFunc since it is already non-blocking.

[shunping]

Thanks!