Fix step-level getWritable() stream timeout on Vercel

[ceolinwill]

Description

Fixes #770

The step-level getWritable() used .pipeTo() to pipe serialized data to the server. .pipeTo() only resolves when the stream closes, but users typically call writer.releaseLock() without closing. This caused the ops promise (passed to waitUntil()) to never resolve, keeping the Vercel function alive until runtime timeout.

PR #678 fixed this for the stream revivers in getStepRevivers/getExternalRevivers by using flushablePipe + pollWritableLock, but the step-level getWritable() (which is what users call via import { getWritable } from 'workflow') was not updated.

This PR applies the same flushablePipe + pollWritableLock pattern to getWritable() so the ops promise resolves as soon as the user releases the writer lock and all pending writes are flushed.

How did you test your changes?

Added packages/core/src/step/writable-stream.test.ts with two tests:

• releaseLock without close — previously hung (reproduces the bug), now resolves
• explicit close — continues to work as before

All existing tests pass (pnpm test in packages/core).

PR Checklist - Required to merge

• 📦 pnpm changeset was run to create a changelog for this PR
  • During beta, we only use "patch" mode for changes. Don't tag minor/major versions.
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• 🔒 DCO sign-off passes (run git commit --signoff on your commits)
• 📝 Ping @vercel/workflow in a comment once the PR is ready, and the above checklist is complete

[changeset-bot]

🦋 Changeset detected

Latest commit: 4ba509e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 16 packages

Name	Type
@workflow/core	Patch
@workflow/builders	Patch
@workflow/cli	Patch
@workflow/next	Patch
@workflow/nitro	Patch
@workflow/vitest	Patch
@workflow/web-shared	Patch
workflow	Patch
@workflow/world-testing	Patch
@workflow/astro	Patch
@workflow/nest	Patch
@workflow/rollup	Patch
@workflow/sveltekit	Patch
@workflow/vite	Patch
@workflow/nuxt	Patch
@workflow/ai	Patch

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[ceolinwill]

@vercel/workflow This PR is ready for review.

[vercel]

@ceolinwill is attempting to deploy a commit to the Vercel Labs Team on Vercel.

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[VaguelySerious]

Thanks, LGTM, running some more validations before merging