[BUG][Dart] Incorrectly handling of `type: string` combined with `format: decimal`

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The Dart generator maps properties with type: string and format: decimal as doubles when parsing JSON. The API returns a string, as described in the schema. Given the type mismatch, the mapValueOfType helper method sets the field to null.

If the field is required, a runtime error occurs because the fromJson method uses the bang operator (!) to assert the field is not null but mapValueOfType returns null.

openapi-generator version

The incorrect mapping of decimal to double has existed since at least 5.4.0. The runtime issue appeared in 6.0.0 with the introduction of the bang operator (!).

I am currently using 6.2.1.

OpenAPI declaration file content or url

https://raw.githubusercontent.com/alpacahq/bkdocs/564d49f52bc91bb180b15b2b900253a1e7c53846/assets/openapi.yaml

Generation Details

java -jar openapi-generator-cli.jar generate -i https://raw.githubusercontent.com/alpacahq/bkdocs/564d49f52bc91bb180b15b2b900253a1e7c53846/assets/openapi.yaml -g dart --global-property apiDocs=false,modelDocs=false -o tmp

Steps to reproduce

1. Generate the client library.
2. Note the use of lastEquity: mapValueOfType<double>(json, r'last_equity') in tmp/lib/model/account_extended.dart.
3. Call AccountExtended.fromJson() with last_equity set to a string value.
4. Observe that the return value for lastEquity is null.

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/10836

Suggest a fix

Consider using an actual Decimal type when parsing decimal values: https://pub.dev/packages/decimal.

Start with the generation command and the generated tmp/lib/model/account_extended.dart, focusing on the lastEquity mapValueOfType call and the referenced OpenAPI schema. Reproduce with a string value for last_equity; done means the generated Dart client handles the declared string/decimal field without returning null or triggering a required-field runtime error.