fix(cloudflare): avoid flush lock self-wait

[dmmulroy]

Summary

• Fix workflow final flushing to use the original/bound waitUntil, matching the request wrapper pattern.
• Pass the workflow execution context into Cloudflare SDK initialization so workflow flushes wait for already-scheduled waitUntil work without scheduling the final flushAndDispose(client) through the same flush-locked context.waitUntil.
• Add a regression test that models the Cloudflare Vitest worker pool draining waitUntil promises with a timeout.

Test plan

cd packages/cloudflare && yarn test:unit test/workflow.test.ts test/flush.test.ts

Result:

Test Files  2 passed (2)
Tests       14 passed (14)

Note: this PR was written by Pi using GPT-5.5 on medium and directly guided/steered and reviewed by Dillon Mulroy dillon@cloudflare.com.

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[isaacs]

LGTM, thanks!