tests: Add tests for guide position preservation on fps change

Yash Bavadiyarequested to merge
xevrion/kdenlive:work/fps_change_test into master

Summary

Adds unit tests to verify that guide markers correctly preserve their temporal position (in seconds) when the project framerate is changed.

These test cases are covering three scenarios:

  • fps increase (25 -> 50 fps): a guide at frame 25 (1.0s) should move to frame 50 (still 1.0s)
  • fps decrease (50 -> 25 fps): a guide at frame 50 (1.0s) should move to frame 25 (still 1.0s)
  • round-trip (25 -> 50 -> 25 fps): guide should return to its original frame position

Implementation notes

To call updateProjectProfile() from the test environment without triggering producer reloads (which crash in headless tests), a static helper was added to KdenliveTests in test_utils.hpp.

Closes #1474 (closed)

Replaces !855 (closed) ,rebranched to follow KDE naming convention as suggested by @emohr.

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