[Condense] Fix bug where bedrock requires a user message first

[canrobins13]

Related GitHub Issue

Closes: #4147

Description

Fixes a bug with condensing where AWS Bedrock requires the first message to be a user message:

A conversation must start with a user message. Try again with a conversation that starts with a user message.

Test Procedure

• Reproduced the bug by condensing and sending an additional message with Bedrock
• Confirmed the fix worked for this case

Type of Change

• 🐛 Bug Fix: Non-breaking change that fixes an issue.
• ✨ New Feature: Non-breaking change that adds functionality.
• 💥 Breaking Change: Fix or feature that would cause existing functionality to not work as expected.
• ♻️ Refactor: Code change that neither fixes a bug nor adds a feature.
• 💅 Style: Changes that do not affect the meaning of the code (white-space, formatting, etc.).
• 📚 Documentation: Updates to documentation files.
• ⚙️ Build/CI: Changes to the build process or CI configuration.
• 🧹 Chore: Other changes that don't modify src or test files.

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Code Quality:
  • My code adheres to the project's style guidelines.
  • There are no new linting errors or warnings (npm run lint).
  • All debug code (e.g., console.log) has been removed.
• Testing:
  • New and/or updated tests have been added to cover my changes.
  • All tests pass locally (npm test).
  • The application builds successfully with my changes.
• Branch Hygiene: My branch is up-to-date (rebased) with the main branch.
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Changeset: A changeset has been created using npm run changeset if this PR includes user-facing changes or dependency updates.
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Screenshots

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Important

Fixes bug in getMessagesSinceLastSummary() to prepend a user message for AWS Bedrock compatibility.

• Behavior:
  • Fixes bug in getMessagesSinceLastSummary() in index.ts where AWS Bedrock requires the first message to be a user message by prepending a user message.
• Tests:
  • Updates index.test.ts to test the new behavior of getMessagesSinceLastSummary() with prepended user message.
• Misc:
  • Adds changeset little-news-warn.md documenting the fix as a major update.

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