toGeminiSchema drops type on JSON Schema nullable type arrays (type: ["string", "null"]), causing 400 on Gemini tool calls #44

Problem

When a tool parameter uses a JSON Schema draft-07 / 2020-12 nullable type array such as type: ["string", "null"] (or ["integer", "null"], ["boolean", "null"], etc.), toGeminiSchema in packages/core/src/transform/gemini.ts checks typeof inputSchema.type === 'string'. Because inputSchema.type is an array, schemaType evaluates to undefined and the emitted Gemini schema drops type completely while keeping description / enum / properties.

OpenCode 2.x built-in and MCP tools emit nullable parameters in this form (for example planId: { type: ["string", "null"] }). When sent to Cloud Code for Gemini models (antigravity-gemini-3.8-flash, antigravity-gemini-4-argon), the request fails with HTTP 400 INVALID_ARGUMENT due to the typeless parameter definition.

Reproduction

Input tool schema property:

{
  "type": ["string", "null"],
  "description": "The plan a note or todo belongs to; null unlinks"
}

Current toGeminiSchema output (missing type and nullable):

{
  "description": "The plan a note or todo belongs to; null unlinks"
}

Expected toGeminiSchema output:

{
  "type": "STRING",
  "nullable": true,
  "description": "The plan a note or todo belongs to; null unlinks"
}

Fix

Normalize single-type nullable arrays (["string", "null"], ["null", "integer"], single-element ["string"], ["null"], and case/duplicate variants) into { type: "<non-null>", nullable: true } before reading inputSchema.type, while leaving multi-type unions (["string", "number"]) untouched for existing validation.