[Backport release/0.0.14] test: skip voice tests without ffmpeg, add ffmpeg to install docs

[github-actions]

Description

Backport of #4105 to release/0.0.14.

[greptile-apps]

Confidence Score: 5/5

Safe to merge; the ffmpeg marker behaves as intended in both local and CI environments.

What we checked:

• Created an isolated pytest probe that uses the repository's collection hook while making ffmpeg unavailable.
• Ran the probe locally with CI unset; the marked test was skipped and pytest exited successfully.
• Reran with CI=1; the same marked test executed and passed rather than being skipped.
• Authored the FFmpeg collection branches harness and captured the final output in the FFmpeg collection branches output log, which shows 1 skipped locally and 1 passed with CI.

Summary

• This change makes voice-input tests skip locally only when ffmpeg is unavailable, while CI still runs the tests so missing dependencies remain visible. It also adds ffmpeg to the Ubuntu installation instructions and fresh-Ubuntu test setup.

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