[Enhancement]Use single instances for Call objects with the same cId

[ipavlidakis]

🎯 Goal

Use single instances for Call objects with the same cId. That helps ensuring that irregardless of the Call object creation point, actions performed to it will be synced across every observer.

📝 Summary

StreamVideo and Call internally use a thread-safe cache that ensures that only a single instance of Call object will exist for the same cId.

🧪 Manual Testing Notes

• Regular join call flows should remain working.

☑️ Contributor Checklist

• I have signed the Stream CLA (required)
• This change follows zero ⚠️ policy (required)
• This change should receive manual QA
• Changelog is updated with client-facing changes
• New code is covered by unit tests
• Comparison screenshots added for visual changes
• Affected documentation updated (Docusaurus, tutorial, CMS)