Fix ffprobe output parsing and unquoted paths in shell commands

[pradeepreddycv]

Two robustness fixes for real-world inputs:

1. utils/probe.py — robust duration parsing. get_video_duration runs ffprobe
   with stderr=subprocess.STDOUT, folding decoder warnings (e.g.
   "[h264 @ ...] time_scale/num_units_in_tick invalid or unsupported") into
   stdout, then calls float(result.stdout) — raising "could not convert string
   to float" on files with noisy-but-valid metadata. Switched to
   stderr=subprocess.PIPE + text=True and parse the numeric line via regex.
   (Same fragility as ffprobe returns error on mpeg-ts inputs #29.)

2. utils/video_reader.py — quote paths in shell commands. load_video_1p0 and
   load_video_1p5 build ffmpeg commands as f-strings run under shell=True with
   the video path unquoted, so any path containing a space breaks with
   "No such file or directory". Wrapped all interpolated paths in shlex.quote().

Test: ran inference on a clip with malformed timing metadata under a directory
containing a space — fails on main, succeeds with these changes.

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