feat: add strict mode to compute_duration and compute_readability fun…

[lpi-tn]

This pull request adds support for a strict mode to the compute_duration and compute_readability functions, allowing callers to force recomputation of metadata even if values already exist. It also introduces comprehensive tests for these behaviors to ensure correctness.

Enhancements to metadata computation:

• Added a strict boolean parameter to both compute_duration and compute_readability in welearn_datastack/modules/computed_metadata.py, enabling forced recomputation of duration and readability scores when set to True.
• Updated the logic in these functions so that, if strict is False and a value is already present in document.details, the original value is retained.

Testing improvements:

• Added new unit tests in tests/test_computed_metadata.py to cover both strict and non-strict behaviors for duration and readability computation, verifying that values are correctly overwritten or preserved based on the strict flag.
• Added tests to confirm that estimated reading time and readability scores are correctly set when no prior value exists.

[Copilot]

Pull request overview

This PR introduces a strict mode parameter to the compute_duration and compute_readability functions, allowing callers to control whether existing metadata values should be recomputed or preserved.

Key Changes:

• Added optional strict boolean parameter (default False) to both metadata computation functions
• Implemented early return logic to preserve existing values when strict=False
• Added comprehensive test coverage for strict and non-strict behaviors

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
welearn_datastack/modules/computed_metadata.py	Added strict parameter and conditional logic to compute_duration and compute_readability functions with documentation
tests/test_computed_metadata.py	Added six new test cases covering strict mode, non-strict mode, and initial value setting for both functions

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