perf(strings): optimize anagram signature using frequency counts (#12927)

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

Replaced the sorting-based signature implementation with a frequency-based
approach using `collections.Counter`. This ensures that the signature
represents both characters and their counts, preventing collisions and
better grouping of true anagrams.

Examples:
- "test" → "e1s1t2"
- "finaltest" → "a1e1f1i1l1n1s1t2"
- "this is a test" → " 3a1e1h1i2s3t3"

Also updated the anagram lookup to use the new frequency-based signatures, making results more accurate and avoiding false positives.

* Refactor anagram function return type to list[str]

* Update anagrams.py

* Update anagrams.py

* Update anagrams.py

* Update anagrams.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

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Co-authored-by: Maxim Smolskiy <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>