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

[github-actions]

Pull request must be merged with a description containing the required fields,

Summary:
Type: Feature/Fix/Cleanup
Test Plan:
Jira:

If there is no jira releated to this change, please put 'Jira: NO-JIRA'.

Description can be changed by editing the top comment on your pull request and making a new commit.

[Copilot]

Pull request overview

This PR updates subtitle clock synchronization to use the pipeline position (via IGstGenericPlayerPrivate::getPosition(m_context.pipeline)) instead of the video sink position, aligning subtitle PTS sync with the pipeline clock domain.

Changes:

• Replace position query source from m_context.videoSink to m_context.pipeline using m_player.getPosition(...).
• Simplify control flow by early-returning when position retrieval fails or subtitle source is missing.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[github-actions]

Coverage statistics of your commit:
Congratulations, your commit improved lines coverage from: 0.0% to 84.3%
Congratulations, your commit improved functions coverage from: 0.0% to 92.5%