fix(embed): preserve replies after empty assistant messages

[mameikagou]

Overview

Fixes #2480.

Ignore empty previous assistant content when applying substring deduplication. This keeps the real reply that follows an empty assistant frame while preserving deduplication for identical and contained replies.

Adds regression coverage through the real embed widget WebSocket message path.

Screenshots

Not applicable. The behavior is covered by executable widget regression tests.

Checklist

• I have read the contribution guide.
• I will sign the CLA when prompted by the bot.
• I have communicated with the project maintainer.
• I have tested the changes and ensured they work as expected.

Tests

• pnpm test:unit (58 passed)
• pnpm lint (0 errors; existing warnings only)
• pnpm exec tsc --noEmit
• pnpm build
• pnpm exec prettier --check ../src/langbot/templates/embed/widget.js tests/unit/widget-message-dedupe.test.mjs

[github-actions]

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[RockChinQ]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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