fix(openrouter): Handle missing usage data in API responses.

[pocketarc]

OpenRouter doesn't always return usage data in API responses. When usage.prompt_tokens or usage.completion_tokens are missing, data_get() returns null, causing a TypeError in the Usage constructor which requires int values.

The Stream handler already handles this correctly. This aligns the Text and Structured handlers with the Stream handler.

Before:

TypeError: Prism\Prism\ValueObjects\Usage::__construct(): Argument #1 ($promptTokens) must be of type int, null given

After:

Missing token values default to 0, no error.