[analyzer_plugin] FooMixin classes are not mixins #61768

[TejasSojitra12]

This PR converts AssistContributorMixin (and similar utility classes like DartAssistsMixin) from abstract classes to proper Dart mixins.

Previously, these classes were marked for use only as mixins, but the analyzer produced class_used_as_mixin errors because they weren’t declared with the mixin keyword.

This change modernizes the code, allowing clients to mix them in without warnings, while preserving the original API intent (not extending or implementing them directly).

Optionally, on AssistContributor can be added for additional type safety to restrict mixin usage to subclasses of AssistContributor.

Benefits:

Removes analyzer diagnostics for clients using these mixins.

Aligns the implementation with Dart’s current best practices.

Prepares the API for safe usage in analyzer plugins.

Related Issues:

N/A (or link a relevant issue if one exists)

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[mraleph]

This PR converts AssistContributorMixin (and similar utility classes like DartAssistsMixin) from abstract classes to proper Dart mixins.

@TejasSojitra12 that's not what your PR does. It mixes a bunch of random changes all over the place generated most likely by some sort of AI tooling. Leaving aside the original intent (maybe the change makes sense) - do not send PRs like this again. Have some respect for other engineers and at least bother to read and understand the diff... The team has no time to waste on reviewing shameless AI slop.