Expose task-level and estimator-level preprocessors as public API

[Copilot]

Plan to expose preprocessor APIs

• Explore repository structure and understand preprocessing flow
• Add public preprocess() method to AutoML class for task-level preprocessing
• Add public preprocess() method to estimator classes for estimator-level preprocessing
• Add tests for the new preprocessor APIs
• Validate tests pass
• Add documentation for the new API methods
• Add example script demonstrating API usage
• Run broader test suite to ensure no regressions
• Address code review feedback
• Fix formatting issues with pre-commit hooks

Summary

This PR successfully exposes the previously internal preprocessing methods as user-facing APIs, addressing issue #228.

Key Changes:

1. AutoML.preprocess(X) - Task-level preprocessing API
2. BaseEstimator.preprocess(X) - Estimator-level preprocessing API
3. Comprehensive documentation and examples
4. Full test coverage (8/8 tests passing)

Latest Changes:

• Fixed formatting issues identified by pre-commit hooks (trailing whitespace, code formatting)
• All tests continue to pass

Original prompt

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This section details on the original issue you should resolve

<issue_title>API and documentation of preprocessors</issue_title>
<issue_description> There are two preprocessors: task-level and estimator-level. The former is automl._preprocess() and the latter is automl.model._preprocess(). The former should be called before the latter. Right now they are not exposed as user-facing APIs.
We can add some user-facing APIs in future version. If you have any ideas for that please feel free to suggest.

Originally posted by @sonichi in #228 (reply in thread)
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes API and documentation of preprocessors #842

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[thinkall]

@copilot, run "pre-commit run --all-files" to fix formatting issues.

[Copilot]

@copilot, run "pre-commit run --all-files" to fix formatting issues.

Done! Pre-commit hooks have been run and formatting issues fixed. Changes include:

• Removed trailing whitespace
• Applied Black code formatting
• Fixed markdown formatting

All tests still pass. Commit: 254ef39