[dart-dio] Support `BigInt` for `"type":"string", "format":"int64"` in Dart generator

Is your feature request related to a problem? Please describe.

Currently any OpenAPI schema property declared as:

"id": {
  "example": "1",
  "format": "int64",
  "type": "string"
}

is generated as a Dart String. This forces users to manually parse large integers themselves, even though Dart has built-in BigInt support and json_serializable can already handle BigInt encoding/decoding.

Describe the solution you’d like

Enhance the Dart generator so that fields with "type": "string", "format": "int64" map to Dart BigInt.

• In models: generate BigInt typed fields.
• In (de)serialization: use BigInt.parse(...) and .toString().
• Optionally expose a generator flag to fall back to String if desired.

Describe alternatives you’ve considered

• Post-generation script to replace all String with BigInt and insert manual (de)serialization logic.
• Manually converting in app code after receiving responses.

Additional context

The json_serializable package already supports BigInt via custom converters. Automating this in the generator would save repeated boilerplate and reduce runtime errors when handling very large 64-bit IDs.

Start by locating the dart-dio generator's type-mapping and model serialization entry points, then inspect how string/int64 schemas are handled. Verify the generated model fields and deserialization/serialization behavior for the example schema; done means those fields use BigInt with the requested conversions, while any fallback option is clearly defined.