fix: Accept usage metadata without a total token count

[aydinfatih]

The API leaves promptTokenCount and totalTokenCount out of usageMetadata in some cases. For example, gemini-2.5-flash with a per-part media resolution returns:

"usageMetadata": {
  "candidatesTokenCount": 10,
  "thoughtsTokenCount": 667,
  "serviceTier": "standard"
}

totalTokenCount was required, so the whole response failed to parse:

TypeError: Gemini\Data\UsageMetadata::__construct(): Argument #2 ($totalTokenCount) must be of type int, null given

Fix

Default a missing totalTokenCount to 0, like promptTokenCount already is. The property type stays int.

Tests

• A response with usage metadata that has neither count parses, and text() works. The test fails without the fix.

Verified against the API: the request above failed on main and works with this change.