Added HEX↔RGBA color utilities and tests (Hacktoberfest 2025) (#1988)

* Added color utility functions: hex_to_rgba, rgba_to_hex, is_valid_hex with tests
* Parametrize hex and RGBA utility tests in `test_utils.py` for improved clarity and coverage
* Enhance hex and RGBA utilities: improve test coverage, validations, and docstrings; refactor shared logic.
* Refactor color input handling with `_normalize_color_input` utility; add hex string support across annotators and enhance test coverage

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Jirka Borovec <6035284+Borda@users.noreply.github.com>