.NET: Support custom audio-to-text endpoints

[LauraGPT]

Motivation and Context

The existing .NET audio-to-text connector can use a custom OpenAIClient, but the standard kernel-builder and dependency-injection paths do not expose a custom endpoint and their API-key overload requires a key. This makes local OpenAI-compatible transcription services unnecessarily difficult to configure.

This change enables self-hosted speech-to-text servers, including FunASR, through the existing IAudioToTextService abstraction without adding provider-specific dependencies.

Closes #14067

Description

• Add a custom Uri endpoint overload to OpenAIAudioToTextService.
• Expose equivalent overloads through IKernelBuilder and IServiceCollection.
• Allow the API key to be omitted for local endpoints.
• Verify that requests use {endpoint}/audio/transcriptions.
• Add a Concepts sample using FunASR at http://localhost:8000/v1 with the sensevoice model.

The implementation follows the existing custom-endpoint pattern used by OpenAIChatCompletionService.

Validation

• OpenAI connector unit tests: 499 passed, 1 existing skip, 0 failed
• Connectors.OpenAI build/package: net10.0, net8.0, netstandard2.0; 0 warnings, 0 errors
• Concepts sample build: 0 warnings, 0 errors
• dotnet format --verify-no-changes: passed
• git diff --check: passed

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I did not break anyone

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Reviewers: 5 | Confidence: 60% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

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[Copilot]

Pull request overview

This PR extends the .NET OpenAI audio-to-text connector to support OpenAI-compatible custom endpoints (e.g., self-hosted transcription servers) via new Uri endpoint overloads and corresponding DI/kernel-builder registrations, with API keys being optional for such endpoints.

Changes:

• Added a custom-endpoint constructor overload to OpenAIAudioToTextService allowing optional API key.
• Added matching AddOpenAIAudioToText overloads for IKernelBuilder and IServiceCollection.
• Added unit tests validating custom endpoint usage (including /audio/transcriptions routing) and a Concepts sample (skipped) demonstrating FunASR usage.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dotnet/src/Connectors/Connectors.OpenAI/Services/OpenAIAudioToTextService.cs	Adds a Uri endpoint constructor overload for OpenAI-compatible audio-to-text endpoints.
dotnet/src/Connectors/Connectors.OpenAI/Extensions/OpenAIServiceCollectionExtensions.cs	Exposes the custom-endpoint overload through IServiceCollection DI registration.
dotnet/src/Connectors/Connectors.OpenAI/Extensions/OpenAIKernelBuilderExtensions.cs	Exposes the custom-endpoint overload through IKernelBuilder.
dotnet/src/Connectors/Connectors.OpenAI.UnitTests/Services/OpenAIAudioToTextServiceTests.cs	Adds a test ensuring custom endpoint requests target {endpoint}/audio/transcriptions without an API key.
dotnet/src/Connectors/Connectors.OpenAI.UnitTests/Extensions/ServiceCollectionExtensionsTests.cs	Adds DI registration test for the custom-endpoint overload.
dotnet/src/Connectors/Connectors.OpenAI.UnitTests/Extensions/KernelBuilderExtensionsTests.cs	Adds kernel-builder registration test for the custom-endpoint overload.
dotnet/samples/Concepts/AudioToText/OpenAI_AudioToText.cs	Adds a skipped Concepts sample showing a local FunASR-style endpoint configuration.

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