Add disposition-aware audio track matching for audio description flavors

[LongingForHome]

Summary

• KFFMpegMediaParser — parses FFprobe disposition flags (e.g. visual_impaired, default) from each audio stream into a new audioDisposition array property on the per-stream KalturaMediaInfo object stored in contentStreams
• KDLAudioMultiStreaming — adds a $disposition property to KDLStreamDescriptor and a new selectStreamByDisposition() method that disambiguates same-language audio tracks by disposition when multiple tracks share the same language code; GetSettings() now accepts flavor tags and automatically injects the required disposition for flavors tagged audio_description
• KDLFlavor — threads $this->_tags through evaluateTargetAudioMultiStream() and GetSettings() so flavor tags are available at stream selection time

Problem

When a source file contains multiple audio tracks with the same language (e.g. a standard eng track and an audio description eng track with visual_impaired disposition), the KDL previously always selected the first matching stream regardless of disposition. This caused:

• Audio description flavors (tagged audio_description) to transcode the wrong track
• Standard flavors to potentially pick up an audio description track

Behavior after this change

Scenario	Result
Standard English flavor + eng/default source track	Selects eng/default ✅
Audio Description flavor (audio_description tag) + eng/visual_impaired source track	Selects eng/visual_impaired ✅
Audio Description flavor, language has no AD source track	Flavor skipped (returns null) rather than using wrong track ✅
Single-track source, any flavor	Existing behaviour preserved ✅
French / Spanish / other language flavors	Unaffected ✅

Test plan

A standalone PHP test script is included that exercises all scenarios against the real sample file stream layout (eng/default, eng/visual_impaired, fra, spa) without requiring a full Kaltura platform:

php tests/test_audio_disposition_matching.php

All 13 assertions pass.

Notes

• No schema changes required — disposition matching is driven entirely by the existing tags field on flavorParams (presence of audio_description tag)
• Backward compatible — single-stream sources and sources without disposition tags follow the existing code path unchanged
• The disposition property on KDLStreamDescriptor can also be set directly via the new-format multiStream JSON ({"audio":{"streams":[{"lang":"eng","disposition":["visual_impaired"]}]}}) for future use cases

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