fix: avoid duplicate SSE close callbacks

[Genmin]

Summary

• guard the SSE response close handler so programmatic close() does not fire onclose twice
• add a real HTTP regression test covering close() ending the SSE response

Fixes #321

Validation

• npm test -- test/server/sse.test.ts -t "should only call onclose once"
• npm test -- test/server/sse.test.ts
• npm run typecheck
• npm run lint
• npm test
• npm run build

[changeset-bot]

🦋 Changeset detected

Latest commit: 1c9fe2c

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

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commit: 1c9fe2c

[Genmin]

Added the missing patch changeset for the v1.x SDK package so the release-metadata blocker should clear on the next checks run. The code checks were already green before this metadata-only update.