feat: implement Gemini File Search APIs (Stores & Documents)

[jfradj]

These commits introduce the new Gemini File Search API

[Copilot]

Pull request overview

This pull request implements the Gemini File Search API, introducing functionality to manage file search stores and documents. The implementation follows the existing codebase patterns for API resource management, adding new contracts, request/response classes, and resource implementations.

Key Changes

• Added FileSearchStores resource with methods for CRUD operations on stores and documents
• Implemented file upload functionality specifically for file search stores using multipart/related requests
• Created comprehensive test coverage for all FileSearchStores operations

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 17 comments.

Show a summary per file

File	Description
src/Contracts/Resources/FileSearchStoresContract.php	Defines interface for file search stores operations - missing strict_types declaration
src/Resources/FileSearchStores.php	Main resource implementation providing CRUD operations for stores and documents
src/Requests/FileSearchStores/*.php	Request classes for create, get, list, delete, and upload operations
src/Requests/FileSearchStores/Documents/*.php	Request classes for document-specific operations
src/Responses/FileSearchStores/*.php	Response classes for parsing API responses
src/Responses/FileSearchStores/Documents/*.php	Response classes for document-related responses
src/Testing/Responses/Fixtures/FileSearchStores/*.php	Test fixtures for response objects
tests/Resources/FileSearchStores.php	Comprehensive test suite covering all operations
src/Client.php	Integration of fileSearchStores() method into main client
src/Contracts/ClientContract.php	Contract update to include fileSearchStores() method
README.md	Documentation with multiple critical errors in API usage examples

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