Rename GetCognitiveServicesEndpoint to GetCognitiveServicesScope and return string

[Copilot]

Methods named GetCognitiveServicesEndpoint() were returning OAuth scopes (e.g., https://cognitiveservices.azure.com/.default), not endpoints. This was misleading and required unnecessary .ToString() conversions when passing to TokenRequestContext.

Changes

• FastTranscriptionRecognizer.cs: Renamed GetCognitiveServicesEndpoint() → GetCognitiveServicesScope(), changed return type from Uri to string
• RealtimeTranscriptionRecognizer.cs: Changed GetCognitiveServicesScope() return type from Uri to string for consistency
• RealtimeTtsSynthesizer.cs: Updated to use non-obsolete GetCognitiveServicesScope() method

Before/After

// Before
var tokenRequestContext = new TokenRequestContext([GetCognitiveServicesEndpoint().ToString()]);

// After
var tokenRequestContext = new TokenRequestContext([GetCognitiveServicesScope()]);

This naming now correctly reflects the method's purpose and eliminates unnecessary type conversions across all Speech tool components.

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