Fix lint

[damccorm]

Mypy didn't like this final change - 015412a - to switch to pipe operators (went undetected because all precommits were down, and the change seemed safe)

Reverting some of it to get lint healthy

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Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a Mypy compatibility issue introduced by the use of pipe operators for union types. By reverting these to the standard typing.Union syntax, the codebase ensures broader support across different static analysis environments.

Highlights

• Type Hint Compatibility: Replaced pipe operator (|) union types with typing.Union to ensure compatibility with older Mypy versions.
• Code Cleanup: Removed unused Agent variable definition and updated type aliases to use string forward references for better type safety.

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