Add assert_includes suggestion to Minitest/AssertMatch cop

[bquorning]

Extends the Minitest/AssertIncludes cop to suggest assert_includes when assert_match is used with simple literal regexes that don't require regex-specific features.

See also rubocop/rubocop-rspec#2169.

Changes

• Adds detection for assert_match calls with regexes that contain only literal characters (no anchors, character classes, quantifiers, alternations, metacharacters, etc.)
• Suggests assert_includes as a more semantic and clearer alternative for simple substring matching
• Makes regexp_parser a direct dependency (already a transitive dependency via rubocop) to properly parse and analyze regex patterns
• Handles escaped literals in regexes (e.g., http:\/\/example\.com → http://example.com)
• Converts simple regexes to their string equivalents with proper quote handling

Examples

# bad
assert_match(/foo/, 'foobar')
assert_match(/http:\/\/example\.com/, response.body)

# good
assert_includes('foobar', 'foo')
assert_includes(response.body, 'http://example.com')

# still uses assert_match (regex features needed)
assert_match(/^foo/, 'foobar')     # has anchor
assert_match(/foo\d+/, 'foobar')   # has quantifier
assert_match(/foo.bar/, 'foobar')  # has metacharacter

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

🤖 Generated with Claude Code

[bquorning]

cc @koic