fix(think): wrap stream-resume ACK fallback in sendIfOpen

[whoiskatrin]

Summary

Follow-up to #1376. That PR hardened agents/chat and @cloudflare/ai-chat against the closed-WebSocket replay race but intentionally left @cloudflare/think for a separate PR. This is that PR.

ResumableStream.replayCompletedChunksByRequestId now returns false both when no completed stream is found and when a sendIfOpen call fails because the socket closed mid-replay. The matching fallback in packages/ai-chat/src/index.ts was updated to use sendIfOpen, but the identical pattern in packages/think/src/think.ts:_handleStreamResumeAck still used a bare connection.send() — so on the close-mid-replay path it would throw TypeError: WebSocket send() after close up through onMessage, which is exactly the failure mode #1376 set out to prevent.

This PR mirrors the sendIfOpen helper from @cloudflare/ai-chat into @cloudflare/think and routes the fallback done: true send through it.

Review & Testing Checklist for Human

• Confirm copying sendIfOpen / isWebSocketClosedSendError into think.ts is preferred over exporting a shared helper from agents/chat (the same duplication already exists in @cloudflare/ai-chat and agents/chat/resumable-stream.ts).
• Sanity check the changeset entry — @cloudflare/think patch — matches the release process you'd expect for this kind of resilience fix.

Notes

• No behaviour change on the happy path: sendIfOpen only swallows TypeError: "WebSocket send() after close" and re-throws everything else.
• Local verification: npm run check, npm run lint, and npx nx run @cloudflare/think:test (289 tests) all pass.

Link to Devin session: https://app.devin.ai/sessions/303193c332f24e08b4c8525adb58e285
Requested by: @whoiskatrin

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[changeset-bot]

🦋 Changeset detected

Latest commit: 0c5e260

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

This PR includes changesets to release 1 package

Name	Type
@cloudflare/think	Patch

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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commit: 0c5e260