feat: Support multimodal function responses

[aydinfatih]

Refs #163

Adds the FunctionResponse fields from the API reference, so a function can return images or documents to the model:

new FunctionResponse(
    name: 'get_product_image',
    response: ['image' => 'product.png'],
    id: $functionCall->id,
    parts: [
        new FunctionResponsePart(inlineData: new Blob(mimeType: MimeType::IMAGE_PNG, data: base64_encode($png))),
    ],
);

Changes

• FunctionResponse::$parts: ordered FunctionResponsePart items with inline media. The existing Blob class is reused for the inline data.
• FunctionResponse::$willContinue and FunctionResponse::$scheduling (FunctionResponseScheduling) for NON_BLOCKING function calls.

Backward compatibility

• New constructor parameters are nullable and appended at the end.
• Existing function responses serialize exactly as before (name, response, id); the new keys are only added when set.
• An unknown scheduling value resolves to null.

Tests

• toArray() without the new fields (unchanged payload) and with them
• from() with all fields, without optional fields and with an unknown scheduling value
• Part round trip with a multimodal function response