Add test case to document cascade-failure behavior in nicediff comparison

[Hell1213]

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• I have read and understood the contributors guide.
• I have checked that the feature I am suggesting isn't already present.
• I have checked that the issue I'm posting isn't already reported.
• I have checked that the issue I'm posting isn't already solved and no duplicates exist in closed issues and in opened issues.
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My familiarity with the project is as follows:

• I have used the platform extensively, but have not contributed previously.

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Proposal

Add a test case to document the current cascade-failure behavior of the nicediff comparison algorithm when subtitle entries are missing.

Motivation

The nicediff module currently lacks test coverage for the cascade-failure scenario. When comparing subtitle files where one file is missing an entry (e.g., entry #2 is missing), the line-by-line comparison algorithm cascades the mismatch to subsequent entries. This behavior should be documented with a test case for:

1. Future Reference: When implementing the frame-offset-tolerant comparison algorithm (planned for GSoC Sample Platform NG project), developers need to understand the current behavior
2. Test Coverage: Improve test coverage for edge cases in subtitle comparison
3. Regression Prevention: Ensure this behavior is maintained or intentionally changed in future updates

Proposed Changes

Add test_cascade_failure_missing_entry() to tests/test_test/test_diff.py that:

• Creates two subtitle lists where one is missing an entry
• Verifies the diff output contains both the missing and mismatched entries
• Documents the current cascade behavior

Benefits

• Better test coverage for nicediff edge cases
• Documentation of current algorithm behavior
• Reference point for future algorithm improvements
• No changes to existing code logic