fix: remove invalid step-start parts from UIMessage conversion

[mguttmann]

Summary

Fixes AI_InvalidPromptError that breaks sessions when compaction is triggered.

Problem

When a session's context overflows and compaction is triggered, the following error occurs:

AI_InvalidPromptError: Invalid prompt: The messages must be a ModelMessage[]. 
If you have passed a UIMessage[], you can use convertToModelMessages to convert them.

This happens because step-start parts are being added to UIMessage objects, but step-start is not a valid UIMessagePart type in the AI SDK's convertToModelMessages() function.

Solution

• Remove step-start from being added to UIMessage parts (it's only used internally for tracking)
• Simplify the result filter since step-start is no longer included

Testing

• Sessions can now compact without errors
• Existing sessions with context overflow can be recovered

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Found a potential related PR:

PR #8888 - fix(session): safeguard message conversion against invalid prompts

• fix(session): safeguard message conversion against invalid prompts (#… #8888
• This PR appears to address the same AI_InvalidPromptError issue with message conversion, which is the core problem your PR fix: remove invalid step-start parts from UIMessage conversion #8949 is fixing.

[mguttmann]

Closing - issue resolved