Fix Google Gemini Pro 2.5 API integration

[sladwig]

Issue Description

When trying to use the Google Gemini Pro 2.5 model, we're getting API errors related to the JSON schema format:

GoogleGenerativeAIFetchError: [GoogleGenerativeAI Error]: Error fetching from https://generativelanguage.googleapis.com/v1beta/models/gemini-pro-2.5:generateContent: [400 Bad Request] Invalid JSON payload received. Unknown name "additionalProperties" at "tools[0].function_declarations[0].parameters": Cannot find field.
Invalid JSON payload received. Unknown name "$schema" at "tools[0].function_declarations[0].parameters": Cannot find field.

The Google Gemini API appears to reject certain JSON Schema fields that we're sending, specifically:

• $schema field
• additionalProperties field

These fields are valid in JSON Schema but not accepted by Google's API implementation.

Steps to Reproduce

bun run build && bun run bin/cli.js --provider google --model gemini-pro-2.5 "Hi"

Expected Behavior

The CLI should successfully connect to the Google Gemini Pro 2.5 model and receive a response.

Proposed Solution

We need to modify the tool schema conversion for Google models to remove the unsupported fields from the JSON schema before sending it to the API.