Fix download_video KeyError, eliminate CDP stutter, and fix 0-byte WebM bug

[MuhammadSamarShehzad]

This PR resolves several critical bugs and significantly improves the performance and reliability of the Element.download_video() method in botasaurus_driver.

1. Fixed Python .format() KeyError crash

Issue: The raw JavaScript payload string contained single curly braces {} around the JS functions. When Python called .format(filename=..., duration=...) on it, it attempted to parse the JS braces as python format variables, throwing a KeyError.
Fix: Escaped all JavaScript syntax braces to double braces {{ and }}, leaving only {filename} and {duration} for the Python formatter.

2. Eliminated 3-Second Playback Stutter (CDP Latency)

Issue: Previously, download_video() paused the video, injected the MediaRecorder setup, and resumed the video using three separate, sequential Python self.apply() CDP calls. The network round-trips caused the user's browser video to visibly stutter and freeze for 2–3 seconds.
Fix: Consolidated the logic into a single JS execution block. The vid.pause() and vid.play() are now executed entirely client-side. This makes the capture transition instantaneous and completely invisible to the user.

3. Fixed Chrome captureStream() 0-Byte Empty Frame Bug

Issue: It is a known Chrome bug that calling captureStream() on an HTML5 video that is actively playing can result in a dead stream with zero frames (resulting in 0-byte corrupted downloads).
Fix: Added a synchronous bypass inside the JS payload: vid.pause() -> vid.captureStream() -> vid.play(). By doing this synchronously in Javascript, we bypass the Chrome bug and guarantee frame capture without interrupting playback.

4. Added Native MP4 Container Support

Issue: The previous code passed invalid MediaRecorder options ({audio:true, video:true}) and forced a .mp4 file extension on a generic octet/stream blob. In Chrome, this defaults to a WebM container, which causes "Video Error" playback failures on macOS QuickTime when mislabeled as an MP4.
Fix: Removed the invalid options and added dynamic MediaRecorder.isTypeSupported('video/mp4') detection. Chrome M107+ natively supports MP4 encoding; if available, it will now encode a true MP4 container. Otherwise, it safely falls back to standard WebM.

Testing:

• Verified that botasaurus no longer crashes with a KeyError during download_video.
• Tested the video capture to confirm the 3-second UI freeze is completely gone.
• Confirmed output files are no longer 0-bytes and play natively on Mac QuickTime.