Add comprehensive file support to AnthropicClient

[Copilot]

This PR implements complete file management capabilities for the Anthropic API, enabling users to upload, manage, and download files through the AnthropicClient.

📁 New File Operations

All file operations requested in the issue have been implemented:

• ✅ Create a file - CreateFileAsync() with multipart form data support
• ✅ List Files - ListFilesAsync() and ListAllFilesAsync() with pagination
• ✅ Get File Metadata - GetFileAsync() to retrieve file information
• ✅ Download a File - DownloadFileAsync() with proper content handling
• ✅ Delete a File - DeleteFileAsync() to remove files

🏗️ Implementation Details

New Models

• AnthropicFile - Represents file metadata with proper JSON serialization
• FileRequest - Handles file upload requests with validation
• FileDeleteResponse - Confirms file deletion operations
• FileDownloadResponse - Contains downloaded file content and metadata

API Client Extensions

Extended IAnthropicApiClient and AnthropicApiClient with file operations following existing patterns:

• Consistent error handling with AnthropicResult<T>
• Full async/await support with cancellation tokens
• Pagination support matching existing List/ListAll patterns

Example Usage

var client = new AnthropicApiClient("your-api-key", new HttpClient());

// Upload a file
var content = File.ReadAllBytes("document.pdf");
var request = new FileRequest(content, "document.pdf", "application/pdf");
var result = await client.CreateFileAsync(request);

// List files
var files = await client.ListFilesAsync();

// Download file content
var download = await client.DownloadFileAsync(fileId);
var fileBytes = download.Value.Content;

// Delete file
await client.DeleteFileAsync(fileId);

🧪 Testing

Comprehensive test coverage includes:

• 22 file-related tests covering all operations
• Unit tests for model serialization/deserialization
• Integration tests with HTTP mocking
• Input validation and error handling tests
• Content-Disposition header parsing tests

🔄 Backwards Compatibility

This is a purely additive change - no existing functionality is modified. All existing tests continue to pass.

Fixes #35.

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