test: expand core test coverage and dedupe mix_tailwinds helpers

[leoafarias]

Related issue

N/A

Description

Expands unit test coverage across mix and mix_generator, and consolidates duplicated widget-testing boilerplate in mix_tailwinds into shared helpers.

Changes

• Added new tests: Equatable (props/hash/diff), typed ContextVariant factories (brightness/breakpoint/not/responsive shorthands), StyleWidget default style resolution, and generator library_scope reference/visibility helpers.
• Expanded existing tests: animation builder/driver, color token integration, variant mixin, widget state variant, generator smoke test, and mix_lint rule tests.
• Extracted shared Flutter widget test helpers into packages/mix/test/helpers/testing_utils.dart.
• Consolidated mix_tailwinds test boilerplate into a shared tailwinds_test_helpers.dart, rewriting div_and_span_test.dart, smoke_test.dart, and tw_config_test.dart to reuse it, and adding div_and_span_margin_test.dart.

Review Checklist

• Testing: Test-only change; added/expanded unit and widget tests, no production code touched.
• Breaking Changes: None — test files only.
• Documentation Updates: N/A
• Website Updates: N/A

Additional Information (optional)

None.