Add Files API resource

[mozex]

Summary

Adds $client->files() with five methods — upload, list, retrieveMetadata, download, delete. Anthropic's Files API lets you upload a document once and reference it by file_id in later Messages calls, which is useful for repeated PDFs or images and for consuming outputs from the code execution tool and Skills.

$file = $client->files()->upload(['file' => fopen('/path/to/doc.pdf', 'r')]);

$response = $client->messages()->create([
    'model' => 'claude-opus-4-6',
    'max_tokens' => 1024,
    'betas' => ['files-api-2025-04-14'],
    'messages' => [[
        'role' => 'user',
        'content' => [
            ['type' => 'text', 'text' => 'Summarise this.'],
            ['type' => 'document', 'source' => ['type' => 'file', 'file_id' => $file->id]],
        ],
    ]],
]);