feat: add EvoLink video provider

[EvoLinkAI]

Summary

Adds evolink_video, a video generation provider backed by EvoLink's async video task API.

The tool follows the existing OpenMontage BaseTool provider pattern and is auto-discoverable by video_selector via capability = \"video_generation\". It supports:

• Seedance 2.0 text-to-video
• Seedance 2.0 image-to-video using public image URLs
• standard/fast model variants
• duration, quality/resolution, aspect ratio, audio toggle, content filter, and web search fields
• submit -> poll GET /v1/tasks/{task_id} -> download MP4 workflow

Also documents EVOLINK_API_KEY in .env.example, README.md, and docs/PROVIDERS.md, and adds unit coverage for metadata, status, payload mapping, selector registration, and the mocked submit/poll/download path.

Validation

env -u all_proxy -u http_proxy -u https_proxy -u ALL_PROXY -u HTTP_PROXY -u HTTPS_PROXY \
  /tmp/openmontage-evolink-venv/bin/python -m pytest tests/tools/test_evolink_video.py -q

Result:

......                                                                   [100%]
6 passed in 0.76s

env -u all_proxy -u http_proxy -u https_proxy -u ALL_PROXY -u HTTP_PROXY -u HTTPS_PROXY \
  /tmp/openmontage-evolink-venv/bin/python -m compileall tools/video/evolink_video.py tests/tools/test_evolink_video.py

Result:

Compiling 'tests/tools/test_evolink_video.py'...

Live EvoLink tool-path smoke test through EvoLinkVideo.execute():

success= True
error= None
model= seedance-2.0-text-to-video
cost_estimate= 0.37
duration_seconds= 209.32
artifacts= ['/tmp/openmontage-evolink-real.mp4']
task_id= task-unified-1781671188-fib1epbg
file_size_bytes= 504418
video_width= 864
video_height= 496
video_codec= h264

Additional output probe:

ffprobe -v error -select_streams v:0 \
  -show_entries stream=codec_name,width,height \
  -show_entries format=duration,size \
  -of default=noprint_wrappers=1 /tmp/openmontage-evolink-real.mp4

Result:

codec_name=h264
width=864
height=496
duration=4.041667
size=504418

[EvoLinkAI]

Live EvoLink validation through the new EvoLinkVideo.execute() path passed.

success= True
error= None
model= seedance-2.0-text-to-video
cost_estimate= 0.37
duration_seconds= 209.32
artifacts= ['/tmp/openmontage-evolink-real.mp4']
task_id= task-unified-1781671188-fib1epbg
file_size_bytes= 504418
video_width= 864
video_height= 496
video_codec= h264

ffprobe on the downloaded artifact:

codec_name=h264
width=864
height=496
duration=4.041667
size=504418

Unit validation:

$ env -u all_proxy -u http_proxy -u https_proxy -u ALL_PROXY -u HTTP_PROXY -u HTTPS_PROXY /tmp/openmontage-evolink-venv/bin/python -m pytest tests/tools/test_evolink_video.py -q
......                                                                   [100%]
6 passed in 0.76s