refactor Result class, remove asOk and asError (#2542)

As discussed in the PR for the Result pattern implementation
(https://github.com/flutter/website/pull/11444) @parlough recommended
that `asError` and `asOk` should be not be used, and instead we should
use proper exhaustiveness checking.

This PR removes the two "convenience" methods and refactors code.

In some cases, it was enough with writing a proper `if` clause, while in
others it was necessary to use a `switch`.

Still, they are present in the `testing` folder, as they can be useful
for testing purposes.

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advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

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compass_app/app/lib/utils/result.dart

@@ -23,12 +23,6 @@ sealed class Result<T> {
 
   /// Creates an error [Result], completed with the specified [error].
   const factory Result.error(Exception error) = Error._;
-
-  /// Convenience method to cast to Ok
-  Ok<T> get asOk => this as Ok<T>;
-
-  /// Convenience method to cast to Error
-  Error get asError => this as Error<T>;
 }
 
 /// Subclass of Result for values