[Vertex AI] Add minItems and maxItems to Schema

[andrewheard]

Added support for specifying the minimum and maximum number of items (minItems / maxItems) to generate in an array Schema. These are two of the fields planned for #14647.

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Code Review

This pull request introduces minItems and maxItems properties to the Schema class, allowing developers to specify the minimum and maximum number of items in an array schema. The changes are well-structured and the new functionality is integrated seamlessly. The addition of a unit test is also a great practice.

Merge Readiness

The code is well-structured, includes a unit test, and introduces a useful feature. I am unable to directly approve this pull request, and recommend that others review and approve this code before merging. I believe this pull request is ready to be merged.