fix(llm): normalize Gemini schema union types

[daniel-kurzynski]

Summary

• normalize array-valued JSON Schema type unions to Gemini-supported anyOf schemas
• preserve scalar type values and existing schema sanitization behavior
• add regression coverage for nested union types

Motivation

Gemini's Schema.type accepts a single scalar enum value, while tool definitions may use standard JSON Schema unions such as:

{"type": ["string", "number", "boolean"]}

Sending such a declaration can cause request translation to fail before the model receives the request. Expressing the same union with anyOf is accepted.

Validation

• cargo test -p llm vertex --lib (14 passed)
• cargo check -p llm --tests
• cargo clippy -p llm --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• replayed the complete 146-tool request successfully with the existing stream_function_call_arguments: true behavior retained

This PR intentionally does not change streaming function-call argument behavior.