feat: modern function-calling config — validated mode, allowed functions, call ID round-trip

[PandelisZ]

What this adds

Support for the current Gemini function-calling API contract as documented at https://ai.google.dev/gemini-api/docs/function-calling.

Three gaps existed in the library that made it impossible to use the modern Gemini function-calling surface correctly:

1. No way to set functionCallingConfig — the model always used AUTO mode implicitly.
2. No allowedFunctionNames — callers couldn't restrict or force a specific function.
3. FunctionCall and FunctionResponse had no id field — Gemini attaches a stable ID to each call; without round-tripping it back in the response the API rejects multi-turn and parallel tool-call flows.

---

Changes

src/tools/model.rs

• FunctionCallingMode enum with variants Auto / Any / None / Validated, serialising to Gemini's SCREAMING_SNAKE_CASE wire format.
• FunctionCallingConfig struct: mode + allowed_function_names: Option<Vec<String>>.
• ToolConfig wrapper that serialises to {"functionCallingConfig": {...}}.
• FunctionCall.id: Option<String> — preserves the ID Gemini attaches to each call.
• FunctionResponse.id: Option<String> — must echo back the matching call ID.
• Builder helpers: FunctionCall::with_id(), FunctionResponse::with_id().

src/generation/builder.rs

• GenerationRequest gains tool_config: Option<ToolConfig> (serialised as toolConfig).
• .tool_config(config) builder method.

src/lib.rs

Re-exports ToolConfig, FunctionCallingConfig, FunctionCallingMode from the crate root.

src/tests.rs

New unit tests:

• test_tool_config_serializes_validated_mode_and_allowed_function_names — verifies the exact JSON wire shape
• test_function_call_with_id_serialization — round-trips FunctionCall.id
• test_function_response_with_id_serialization — round-trips FunctionResponse.id
• Extended test_multi_turn_content_structure to verify Content::function_call_with_thought propagates thoughtSignature to the Part

---

Usage example

use gemini_rust::{ToolConfig, FunctionCallingConfig, FunctionCallingMode};

// Force the model to call a specific function
let request = GenerationRequest::new(model, contents)
    .tools(vec![my_tool])
  .tool_config(ToolConfig {
        function_calling_config: Some(FunctionCallingConfig {
         mode: FunctionCallingMode::Any,
            allowed_function_names: Some(vec!["get_weather".to_string()]),
        }),
        ..Default::default()
    });

// Round-trip the call ID in the response
let call_id = function_call.id.clone();
let response = FunctionResponse::new("get_weather", result_json)
    .with_id(call_id.unwrap());

---

All 11 existing + new unit tests pass.

[PandelisZ]

Hi @flachesis

Thanks for the approval. I saw you merged in main and the CI failed so I've rebased on master and fixed the issues.

So this is now mergable and re-tested.

I hope that helps and feel free to merge when you're happy with it