fix(provider): Copilot Gemini models fail to produce structured tool calls #15315

Description

Copilot Gemini Flash models (e.g. gemini-3-flash-preview) cannot make structured tool calls. The model outputs plain-text descriptions like [tool_call: mirror-url ...] instead of structured tool_calls in the response.

Root cause

sanitizeGemini in ProviderTransform.schema() matches Copilot Gemini models via api.id.includes("gemini"). This transform is designed for Google's native Gemini API, but Copilot models use an OpenAI-compatible endpoint — the Copilot API handles Gemini schema conversion internally.

Two post-v1.1.49 commits added aggressive schema transforms that produce double-transformed schemas the Copilot translation layer can't handle:

• fix: handle nested array items for Gemini schema validation #11952 — default array item types to "string"
• fix(provider): strip properties/required from non-object types in Gemini schema #11888 — strip properties/required from non-object types

This breaks tool calling for Copilot Gemini models. Copilot Claude/GPT models are unaffected (they don't match includes("gemini")).

Steps to reproduce

1. Authenticate with GitHub Copilot
2. Select a Gemini model (e.g. gemini-3-flash-preview)
3. Send a message that should trigger a tool call
4. Observe that the model outputs a plain-text tool call description instead of a structured tool_calls response

Expected behavior

The model should produce structured tool_calls in the response, the same as Copilot Haiku or GPT models.

Environment

• OpenCode version: any post-v1.1.49
• Provider: github-copilot
• Model: gemini-3-flash-preview