Preserve input time base in concatenate_video_files function

[irisTa56]

What this does

Fixes the second error of #2158.
Effectively reverts #2133.

We cannot set time_base to the created stream without PyAV-Org/PyAV#1934, so I updated the minimum version of PyAV.
I believe the updated requirement will not cause problems since requirements-*.txt already specifies av==15.0.0, but there might be use cases requiring PyAV<15 that I am not aware of.

How it was tested

With the following script (and some video file named test.mp4):

import shutil
from pathlib import Path

import av
from lerobot.datasets.video_utils import concatenate_video_files


def show_time_bases(path: Path):
    with av.open(str(path)) as container:
        stream = container.streams.video[0]
        print(f"{path}: time_base: {stream.time_base}")


test_data = Path("test.mp4")
assert test_data.exists()

# Videos will be concatenated into path0, step by step.
path0 = Path("path0.mp4")
path1 = Path("path1.mp4")
path2 = Path("path2.mp4")

for path in [path0, path1, path2]:
    path.unlink(missing_ok=True)
    shutil.copy(test_data, path)
    show_time_bases(path)

concatenate_video_files([path0, path1], path0)

print("After first concatenation:")
show_time_bases(path0)

concatenate_video_files([path0, path2], path0)

I got:

$ python test.py
path0.mp4: time_base: 1/10240
path1.mp4: time_base: 1/10240
path2.mp4: time_base: 1/10240
After first concatenation:
path0.mp4: time_base: 1/10240

I appreciate any advice about writing unit tests if needed.
I could not find existing tests for video_utils module.

How to checkout & try?

Same as the above.
Code at bf3c874 reproduces the problem as follows:

$ python test.py          
path0.mp4: time_base: 1/10240
path1.mp4: time_base: 1/10240
path2.mp4: time_base: 1/10240
After first concatenation:
path0.mp4: time_base: 1/90000
Application provided invalid, non monotonically increasing dts to muxer in stream 0: 513000 >= 59392
Traceback (most recent call last):
  File "/Users/takayuki/Documents/_repos/lerobot/_tmp/test.py", line 32, in <module>
    concatenate_video_files([path0, path2], path0)
  File "/Users/takayuki/Documents/_repos/lerobot/src/lerobot/datasets/video_utils.py", line 467, in concatenate_video_files
    output_container.mux(packet)
  File "av/container/output.py", line 375, in av.container.output.OutputContainer.mux
  File "av/container/output.py", line 399, in av.container.output.OutputContainer.mux_one
  File "av/container/core.pyx", line 303, in av.container.core.Container.err_check
  File "av/error.pyx", line 424, in av.error.err_check
av.error.ValueError: [Errno 22] Invalid argument: '/var/folders/12/z2mncvk52d75lblxw5wmpryw0000gn/T/tmpvto2jb97.mp4'; last error log: [mp4] Application provided invalid, non monotonically increasing dts to muxer in stream 0: 513000 >= 59392

[Copilot]

Pull Request Overview

This PR fixes a video concatenation issue where the time base was not being preserved when concatenating video files, causing invalid DTS (decode timestamp) errors. The fix explicitly sets the time_base parameter when adding streams and updates the minimum PyAV version requirement to 15.0.0.

• Preserves input time base in video concatenation to prevent timestamp conflicts
• Updates minimum PyAV version requirement from 14.2.0 to 15.0.0 to support the time_base parameter
• Adds detailed comments explaining the time base handling logic

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/lerobot/datasets/video_utils.py	Added explicit time_base parameter and explanatory comments to preserve input stream time base during concatenation
pyproject.toml	Updated minimum PyAV version from 14.2.0 to 15.0.0 to support the time_base parameter

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[irisTa56]

Closing as this change has been included in #2100.