Part of #1592

Scope

Implement native function calling for Gemini via tools + functionDeclarations.

Files to Modify

• crates/zeph-llm/src/gemini.rs -- implement chat_with_tools(), tool definition conversion, tool result handling

Key Implementation Details

Tool Definition Conversion (Zeph -> Gemini)

Zeph ToolDefinition maps to Gemini functionDeclarations:

• JSON Schema type names must be UPPERCASED: string -> STRING, object -> OBJECT, etc.
• $ref / $defs references must be inlined (Gemini does not support JSON Schema references)
• Nested in tools[0].functionDeclarations[] (not flat like OpenAI)

Tool Call Parsing (Gemini -> Zeph)

Gemini returns functionCall as a part in candidates[0].content.parts[]:

{ "functionCall": { "name": "tool_name", "args": {...} } }

Map to ToolUseRequest { id: generated_uuid, name, input: args }.
Note: Gemini does not return a tool call ID -- generate a UUID locally.

Tool Result Format (Zeph -> Gemini)

Tool results are sent as functionResponse parts in a user message:

{ "role": "user", "parts": [{ "functionResponse": { "name": "...", "response": {...} } }] }

Tool Config

Send toolConfig.functionCallingConfig.mode as "AUTO" (default).

Acceptance Criteria

• supports_tool_use() returns true
• chat_with_tools() sends correctly formatted function declarations
• Single and multi-tool call responses parsed into ChatResponse::ToolUse
• Tool results sent back as functionResponse parts
• JSON Schema type names uppercased in conversion
• $ref / $defs inlined in schema conversion
• UUID generated for tool call IDs (Gemini does not provide them)
• Unit tests for tool format conversion edge cases