.NET: Add GitHub Copilot BYOK sample

[droideronline]

Motivation & Context

GitHub Copilot supports BYOK (Bring Your Own Key — https://docs.github.com/en/copilot/how-tos/copilot-sdk/auth/byok), letting requests be routed through a custom OpenAI/Azure/Anthropic-compatible endpoint instead of the default GitHub Copilot backend. SessionConfig.Provider is already passed straight through by GitHubCopilotAgent (see CopyResumeSessionConfig), but there was no sample demonstrating the scenario end-to-end.

Description & Review Guide

• What are the major changes? Adds a new Agent_With_GitHubCopilot_BYOK sample project under dotnet/samples/02-agents/AgentProviders/github-copilot/, showing how to set SessionConfig { Model, Provider = new ProviderConfig { Type, WireApi, BaseUrl, ApiKey, ModelId } } to route through a custom endpoint, reading BYOK_BASE_URL/BYOK_API_KEY/BYOK_MODEL_ID from the environment. Wires the project into agent-framework-dotnet.slnx, adds a row to AgentProviders/README.md, and registers a SampleDefinition in eng/verify-samples/AgentsSamples.cs (skipped automatically when the required env vars are unset).
• What is the impact of these changes? Documentation/sample only — no changes to library code.
• What do you want reviewers to focus on? Whether the ProviderConfig properties used match current guidance.
• Note: I could not build locally because this repo pins the .NET 10 SDK and only .NET 8 was available in my environment. I instead verified every type, property, and namespace used (ProviderConfig, SessionConfig.Provider, SessionConfig.Model) against the github/copilot-sdk source at tag v1.0.5, which matches the version pinned in Directory.Packages.props. Please run a build/CI pass to confirm.

Related Issue

Fixes #

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change.

[Copilot]

Pull request overview

Adds a new .NET sample demonstrating GitHub Copilot SDK BYOK (Bring Your Own Key) usage via SessionConfig.Provider, so developers can route agent requests through their own OpenAI-compatible endpoint rather than the default GitHub Copilot backend.

Changes:

• Added a new Agent_With_GitHubCopilot_BYOK sample (README + runnable Program.cs) that reads BYOK_BASE_URL / BYOK_API_KEY / BYOK_MODEL_ID and configures ProviderConfig.
• Registered the sample in the .NET solution (agent-framework-dotnet.slnx) and in the sample verifier (eng/verify-samples/AgentsSamples.cs) with env-var gating.
• Documented the sample in dotnet/samples/02-agents/AgentProviders/README.md.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
dotnet/samples/02-agents/AgentProviders/README.md	Adds the BYOK sample entry to the AgentProviders index.
dotnet/samples/02-agents/AgentProviders/github-copilot/Agent_With_GitHubCopilot_BYOK/README.md	Documents how to run/configure the BYOK Copilot provider sample.
dotnet/samples/02-agents/AgentProviders/github-copilot/Agent_With_GitHubCopilot_BYOK/Program.cs	Implements the BYOK session/provider configuration and streaming run loop.
dotnet/samples/02-agents/AgentProviders/github-copilot/Agent_With_GitHubCopilot_BYOK/Agent_With_GitHubCopilot_BYOK.csproj	Adds the new sample project referencing Copilot SDK + AF Copilot adapter project.
dotnet/eng/verify-samples/AgentsSamples.cs	Registers the new sample for verification with required/optional env vars.
dotnet/agent-framework-dotnet.slnx	Includes the new sample project in the solution.

[droideronline]

Thanks for the review, @rogerbarreto! All the feedback is addressed:

• Provider type is now configurable via BYOK_PROVIDER_TYPE (default openai) instead of being hardcoded, since the sample already documents Azure/Anthropic support.
• Dropped the "OpenAI-compatible" wording everywhere it incorrectly implied Anthropic — reworded to "your own endpoint" (README, sample header comment, and the AgentProviders/README.md entry).
• Moved the "About BYOK" explainer to the top of the README so the term is introduced before it's used.
• Finished applying the WireApi/ModelId comment suggestions.

Mirrored the same fixes into the Python sample (#7336) for consistency.

Please take another look when you get a chance. Thanks!