Fix VideoCacheThread::isReady() when near the max frame of a timeline

[jonoomph]

Fix VideoCacheThread::isReady() so preroll readiness is clamped by frames actually available to the timeline boundary (end/start), preventing playback from stalling in PLAY near the end.

Also, add regression coverage in tests/VideoCacheThread.cpp (isReady: clamps preroll requirement at timeline boundaries) for forward-near-end and backward-near-start cases.

[jonoomph]

OpenShot does not recognize that it has reached at the end of the timeline and stays in "Play"/"Preview" mode.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.69%. Comparing base (57c1fb2) to head (fb41a3c).

Files with missing lines	Patch %	Lines
src/Qt/VideoCacheThread.cpp	80.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1036      +/-   ##
===========================================
+ Coverage    59.59%   59.69%   +0.09%     
===========================================
  Files          206      206              
  Lines        20511    20567      +56     
===========================================
+ Hits         12224    12277      +53     
- Misses        8287     8290       +3     

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