fix(strings): use frequency-based signature for anagrams

[sowndappan5]

Summary

Replaced the sorting-based signature implementation with a frequency-based
approach using collections.Counter.

Changes

• Updated signature() to produce frequency-based keys
• Ensured better grouping of true anagrams, preventing collisions
• Example outputs:
  • "test" → "e1s1t2"
  • "finaltest" → "a1e1f1i1l1n1s1t2"
  • "this is a test" → " 3a1e1h1i2s3t3"

Tests

• All tests passed locally after modification
• Verified with multiple word examples

Breaking Changes

• signature() output format changed from sorted-letters to frequency-based
• Downstream code/tests relying on old format may break

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
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• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".