Ruby: Implement Herb/IncludesAssertion RuboCop cop

[marcoroth]

This pull request implements a custom RuboCop cop that flags assert_includes and refute_includes in the test suite, so new tests reach for the assert_*_snapshot helpers instead of asserting against a substring of the output.

Herb's tests snapshot the whole compiled or rendered output, which keeps a failure showing the entire diff instead of a single matched fragment. This cop keeps that convention enforced as new tests are added.

# bad
assert_includes result.errors.first.message, "unexpected token"

# good
assert_parsed_snapshot(source)

The cop reports:

Do not use `assert_includes`. Use one of the `assert_*_snapshot` helpers instead.

[github-actions]

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commit: 5e3d905