fix(gemini): strip propertyNames from tool schemas

[pascalwhoop]

Summary

Strip propertyNames from Gemini tool schemas to avoid Google API 400 errors for unsupported function declaration fields.

Context

Gemini function declarations use an OpenAPI-style schema subset and reject some JSON Schema keywords. Tool schemas that include propertyNames can currently fail with errors like:

I found this when trying to get 3.5-flash going

Invalid JSON payload received. Unknown name "propertyNames" at 'tools[0].function_declarations...'

additionalProperties and other unsupported keywords are already removed by the Gemini schema sanitizer; this adds the missing propertyNames case.

Changes

• Add propertyNames to the unsupported keyword list removed from Gemini schemas
• Update the sanitizer comment to document the removal
• Add a regression test that verifies nested propertyNames entries are absent from the serialized sanitized schema

[CLAassistant]

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