fix: fail dts builds when the worker dies without a result

[cpruijsen]

Summary

--dts listens for 'error' and 'exit' on the dts worker and rejects with dts build worker exited with code ${code} without reporting a result. A DTS pass that produced nothing now fails the build.

The parent promise used to settle only on worker 'message' ('success' / 'error'). If the worker died without posting a result, that promise stayed pending, the event loop drained, and tsup exited 0 with JS in dist/ and no .d.ts files. Build caches then stored that artifact as a success; consumers later saw TS7016.

Closes #1410

Decision

Both 'error' and 'exit' handlers, reject(new Error(...)), no settled flag. Alternative: PrettyError (message-only CLI output) and/or a boolean so 'exit' after terminate() is ignored explicitly.

This is the patch the issue verified on 8.5.1, and it matches the existing dts-worker failure reject(new Error('error occurred in dts build')). Rejecting an already-resolved promise is a no-op; existing --dts tests still pass. Can switch to PrettyError or a settled guard.

Test plan

• vitest run test/dts.test.ts -t "dts worker exiting without a result" fails without the handlers (exit 0, no .d.ts), passes with them (exit 1, error names the worker)
• vitest run test/dts.test.ts all 13 dts tests pass (healthy declaration emit unchanged)
• vitest run 96 tests passed locally (Node 26, darwin)
• CI: ubuntu + windows, Node 18/20/22

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