ClientFake does not support testing startChat() + sendMessage() flow

[saskia78-byte]

Description

When using ClientFake to test code that uses startChat() followed by sendMessage(),
a TypeError is thrown because GenerativeModelTestResource::startChat() has a return
type of ChatSession (a final class), but ClientFake tries to return a
GenerateContentResponse from its response queue instead of a ChatSession.

Steps to reproduce

$fakeGeminiClient = new ClientFake([
    GenerateContentResponse::fake(),
]);

$chat = $fakeGeminiClient
    ->generativeModel('gemini-2.5-flash')
    ->startChat();

$response = $chat->sendMessage('Hello');

Error

Gemini\Testing\Resources\GenerativeModelTestResource::startChat():
Return value must be of type Gemini\Resources\ChatSession,
Gemini\Responses\GenerativeModel\GenerateContentResponse returned

Expected behavior

ClientFake should support testing the full startChat() + sendMessage() flow,
not only generateContent().

Environment

• PHP 8.3.28
• google-gemini-php/client (latest)
• PHPUnit 12.5.8
• Symfony 6.4