feat: add audio sample rate adjustment to built-in presets

[bajankristof]

Refs: ARC-11095

[Copilot]

Pull Request Overview

This PR adds audio sample rate adjustment functionality to FFMPEG presets, similar to existing frame rate handling. It ensures audio is encoded at appropriate sample rates by setting a default of 48kHz and using the closest standard sample rate when input rates are lower than targets.

• Audio sample rate parameter added to all built-in presets with 48kHz default
• New audio sample rate adjustment logic implemented with standard rate fallbacks
• Comprehensive test coverage added for various audio sample rate scenarios

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
lib/ffmpeg/command_args.rb	Implements audio sample rate adjustment logic with standard rates array
lib/ffmpeg/presets/h264.rb	Adds audio_sample_rate parameter to all H264 preset methods and class
lib/ffmpeg/presets/aac.rb	Adds audio_sample_rate parameter to AAC preset methods and class
lib/ffmpeg/presets/dash/h264.rb	Updates DASH H264 presets to pass audio_sample_rate parameter
lib/ffmpeg/presets/dash/aac.rb	Updates DASH AAC presets to pass audio_sample_rate parameter
spec/ffmpeg/command_args_spec.rb	Adds comprehensive test coverage for audio sample rate adjustment
lib/ffmpeg/version.rb	Version bump to 8.1.0-beta.4
CHANGELOG	Documents the new audio sample rate features

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