fix(gemini): sanitize schema keywords in function responses to prevent 400

[AhooraZen]

Problem

When tool output (e.g. from webfetch or JSON schema files) contains keys like $ref, $defs, #, or /, tryParseJSON parses them into the functionResponse.response object.

Google Gemini / Antigravity protobuf parser treats these keys as schema references rather than payload data, failing with:

{"error":{"code":400,"message":"The referenced name `#/$defs/Config` in function_response.response does not match to a display_name in the function_response.parts.","status":"INVALID_ARGUMENT"}}

Solution

• Added sanitizeFunctionResponseResult in open-sse/translator/formats/gemini.js to recursively sanitize forbidden schema characters ($, #, /, definitions) in tool result objects before constructing functionResponse.
• Added unit test in scripts/test-gemini-tool-response-sanitization.mjs.