[local_auth] Adopt structured errors - platform interface

[stuartmorgan-g]

Platform interface portion of #9981

• Adds the new LocalAuthException class for structured errors.
• Deprecates AuthenticationOptions.useErrorDialogs

Part of:

• [local_auth] Improve exception handling of local_auth plugin flutter#113687
• [local_auth] Remove useErrorDialogs flutter#175125

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[gemini-code-assist]

Code Review

This pull request introduces structured error handling to the local_auth platform interface by adding a new LocalAuthException class and a LocalAuthExceptionCode enum. This is a valuable improvement as it allows for more granular and consistent error handling across platforms. The contract of the authenticate method has been clarified in the documentation to reflect these changes. Additionally, the useErrorDialogs option in AuthenticationOptions is now deprecated. The changes are well-structured and include necessary updates to tests, the changelog, and pubspec. I have one minor suggestion to enhance the new exception class for better debuggability.

[gemini-code-assist]

For better readability and consistency with Dart conventions, it's good practice for toString() to return a string that resembles a constructor call with named parameters. Using code.name for the enum value will also produce a cleaner output, which is helpful for debugging.

Suggested change

	String toString() => 'LocalAuthException(code $code, $description, $details)';
	String toString() => 'LocalAuthException(code: ${code.name}, description: $description, details: $details)';

[stuartmorgan-g]

I adjusted to follow the Flutter style for toString, using objectRuntimeType, which I had forgotten to do. Flutter style doesn't do named arguments for everything, but I did add the .name for code.