fix(ai-chat): sanitize tool schemas for Gemini function_declarations

[datlechin]

Problem

Sending a message to Gemini with AI Chat tools enabled fails with HTTP 400:

Invalid JSON payload received. Unknown name "additionalProperties" at 'tools[0].function_declarations[0].parameters': Cannot find field.
Invalid JSON payload received. Unknown name "type" at 'tools[0].function_declarations[1].parameters.properties[2].value': Proto field is not repeating, cannot start list.

Root cause: ChatToolSchemaBuilder produces a JSON Schema with:

• additionalProperties: false on every object schema
• type: ["string", "null"] for fields marked optional: true

Both forms work for OpenAI and Claude, but Gemini's function_declarations API rejects them. Gemini's schema is closer to Protocol Buffers — strict single types, no additionalProperties, optional fields use a nullable: true flag instead.

Fix

New static GeminiProvider.sanitizeSchemaForGemini(_:):

• Recursively strips additionalProperties at any depth
• Rewrites type: [X, "null"] to type: X plus nullable: true
• Leaves enums, required, description, and other fields untouched

Applied to each tool's inputSchema before serializing into parameters. OpenAI/Claude paths are untouched — they still receive the unmodified shared schema.

Test plan

• 5 new tests in GeminiProviderEncodingTests.swift covering: strip additionalProperties, rewrite optional types, preserve non-nullable types, recurse into array items, preserve enums and required
• Manual: configure Gemini provider, send a message with tools enabled in AI Chat, verify no 400 errors
• OpenAI and Claude still work (no shared schema change)