feat(azure): add private_endpoint support to Azure TTS services

[brunomartinez-lmi]

Summary

Add private_endpoint parameter to AzureTTSService and AzureHttpTTSService, enabling connection via Private Link or custom domain endpoints.

Motivation

AzureSTTService already supports a private_endpoint parameter (added in PR #3967 and refined in PR #3974), but the TTS services still require a region-based endpoint. Users deploying Azure Speech resources with custom domains or Private Link must currently subclass and monkey-patch the SpeechConfig after initialization.

This PR brings TTS to parity with STT by passing private_endpoint directly to Azure SDK's SpeechConfig(endpoint=...) constructor — the same pattern used in the STT service.

Changes

• Added private_endpoint parameter to AzureBaseTTSService._init_azure_base()
• Added private_endpoint parameter to AzureTTSService.__init__()
• Added private_endpoint parameter to AzureHttpTTSService.__init__()
• Updated start() methods to create SpeechConfig with endpoint= when private_endpoint is provided (matching the STT implementation pattern)
• Made region optional (required only when private_endpoint is not provided)
• Added validation: raises ValueError if neither region nor private_endpoint is given
• Added warning when both are provided (region is ignored, matching STT behavior)

Usage

# Using private endpoint (Private Link or custom domain)
tts = AzureTTSService(
    api_key="...",
    private_endpoint="https://my-resource.cognitiveservices.azure.com/",
    settings=AzureTTSService.Settings(voice="en-US-SaraNeural"),
)

# Existing region-based usage continues to work unchanged
tts = AzureTTSService(
    api_key="...",
    region="eastus",
    settings=AzureTTSService.Settings(voice="en-US-SaraNeural"),
)

References

• Related: PR Fix Azure STT SpeechConfig with private endpoint #3967 (Fix Azure STT SpeechConfig with private endpoint)
• Related: PR Make Azure STT region optional when private_endpoint is used #3974 (Make Azure STT region optional when private_endpoint is used)
• Azure docs: https://learn.microsoft.com/en-us/azure/ai-services/speech-service/speech-services-private-link

🤖 generated by copilot, pre-reviewed by me