Add betas parameter for per-request beta feature opt-in

[mozex]

Adds a first-class betas parameter that any resource method accepts and converts into a per-request anthropic-beta header. Lets us enable beta features without forcing users to manually set the header on the factory.

$response = $client->messages()->create([
    'model' => 'claude-opus-4-6',
    'max_tokens' => 1024,
    'messages' => [...],
    'betas' => ['extended-cache-ttl-2025-04-11'],
]);

The SDK strips betas out of the parameters before serialization, so nothing leaks into the JSON body or the query string. It becomes a comma-separated anthropic-beta header on that one request only.

Key behaviors

• Extraction, not pass-through. betas is the one parameter that's actively consumed by the SDK. Factory methods on Payload (create, list, retrieve, modify, upload) pull it out; the request body never contains it.
• Auto-injection for resources. A new Payload::withBetas(array) method lets resources inject the beta they require (e.g. a future Files::upload() will add files-api-2025-04-14 automatically). Users don't have to type the version string for resources they're already calling.
• Global + per-request merge. If you set anthropic-beta via withHttpHeader() and also pass betas: [...] on a call, the request sends both, de-duplicated in order of appearance.
• Non-string entries filtered out. betas: ['files-api-2025-04-14', 42, null, ''] sends just the one valid string.
• Empty array is a no-op. No anthropic-beta header is added if betas is present but empty.