refactor: clean up internal schema wrangling code in `KurtVertexAI`

[jemc]

This commit makes some of that code more readable/maintainable, and removes some usage of any

[github-actions[bot]]

:tada: This PR is included in version @formula-monks/kurt-vertex-ai-v1.4.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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