Get the pipeline position instead of video sink position when syncing the subtitles

[skywojciechowskim]

Summary: Get the pipeline position instead of video sink position when syncing the subtitles
Type: Fix
Test Plan: UT/CT, Fullstack
Jira: RDKEMW-21216

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[Copilot]

Pull request overview

This PR fixes subtitle synchronization by using the pipeline position (via IGstGenericPlayerPrivate::getPosition) instead of querying the video sink position when emitting the current-pts event to the subtitle source.

Changes:

• Query playback position from the pipeline (m_context.pipeline) rather than m_context.videoSink.
• Early-return when position retrieval fails (instead of continuing with a default/previous value).
• Keep sending a current-pts custom downstream OOB event to the subtitle appsrc for sync.

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Coverage statistics of your commit:
Lines coverage stays unchanged and is: 84.3%
Functions coverage stays unchanged and is: 92.7%