fix: resolve video save path without os.chdir (fixes #378)

[Haris-bin-shakeel]

Summary

Fixes #378 by removing os.chdir() from VideoMaker.make_video() and switching to explicit absolute paths throughout the video pipeline.

Root Cause

make_video() called os.chdir(self.save_fld) so ffmpeg could locate the output file using only a bare filename. The CWD was restored at the end — but only on the happy path. Any exception during rendering left the process CWD permanently mutated, causing subsequent path operations in the caller's script to silently resolve to the wrong location. This explains the intermittent nature of the bug.

_video.py compounded this by passing only a filename to ffmpeg, making output placement entirely dependent on global CWD state.

Changes

• brainrender/video.py: removed curdir = os.getcwd() and both os.chdir() calls; Video() now receives a fully-resolved absolute path via self.save_fld.resolve(); compress() updated to match
• brainrender/_video.py: ffmpeg input dir and output file both quoted to handle paths with spaces
• tests/test_video.py: assertions updated to validate resolved absolute paths
• tests/test_video.py::test_compress PASSED

Verification

tests/test_video.py::test_video PASSED
tests/test_video.py::test_video_custom PASSED
tests/test_video.py::test_animation PASSED

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.90%. Comparing base (2322e0f) to head (f6f63a9).

Files with missing lines	Patch %	Lines
brainrender/video.py	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
- Coverage   89.07%   88.90%   -0.17%     
==========================================
  Files          27       27              
  Lines        1281     1280       -1     
==========================================
- Hits         1141     1138       -3     
- Misses        140      142       +2     

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[Haris-bin-shakeel]

Added test_compress to cover the 4 previously uncovered lines in
compress() mocks os.system and validates that ffmpeg receives
fully-resolved absolute paths for both input and output.
test_compress passes locally.