[dart-dio] The referenced name (extras) isn't visible in scope.

[AndrewDongminYoo]

This pull request removes incorrect references to the extras property within the Dart-Dio code generator templates. Currently, the generated code attempts to use a non-existent extras field on dio.Options, leading to compilation errors. By removing these references, the generated code will correctly compile against the current dio version.

What was done:

• Updated the Dart-Dio client code generation templates to no longer output extras references.
• Verified that generated code compiles and runs without reference errors.
• Regenerated and tested sample outputs to ensure compatibility.

Why this is needed:

• Prevents compilation errors due to non-existent fields.
• Ensures generated Dart-Dio clients are fully compatible with dio's current API.

Issue reference (if any):

• (If there's an open issue related to this, reference it here. Otherwise, remove this line.)

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PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

./mvnw clean package || exit
./bin/generate-samples.sh ./bin/configs/\*.yaml || exit
./bin/utils/export_docs_generators.sh || exit

(For Windows users, please run the script in Git BASH)
Commit all changed files.

• Ensure all generated samples match expected outputs to pass CI checks.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

[AndrewDongminYoo]

@jaumard @josh-burton @amondnet @sbu-WBT @kuhnroyal @agilob @ahmednfwela

[ahmednfwela]

LGTM @wing328