Detect VS Code Copilot terminal via COPILOT_AGENT in LLM telemetry detection

[Copilot]

The LLM environment detector was not identifying Copilot terminal sessions in VS Code. VS Code now exposes COPILOT_AGENT, so Copilot usage could be missed in telemetry without this update.

• LLM telemetry detection

  • Extended the copilot rule in LLMEnvironmentDetectorForTelemetry to treat COPILOT_AGENT as a Copilot signal, alongside existing Copilot env vars.
  • Updated the inline rule comment to reflect the new detection surface.

• Targeted test coverage

  • Added a telemetry test case asserting COPILOT_AGENT=1 resolves to "copilot".

new EnvironmentDetectionRuleWithResult<string>("copilot", new AnyMatchEnvironmentRule(
    new BooleanEnvironmentRule("GITHUB_COPILOT_CLI_MODE"),
    new AnyPresentEnvironmentRule("GH_COPILOT_WORKING_DIRECTORY", "COPILOT_CLI", "COPILOT_AGENT")))

[Copilot]

Pull request overview

Updates the SDK’s LLM telemetry environment detection so VS Code Copilot terminal sessions are recognized via the newly exposed COPILOT_AGENT environment variable, preventing Copilot usage from being missed in telemetry.

Changes:

• Extended the Copilot detection rule in LLMEnvironmentDetectorForTelemetry to treat COPILOT_AGENT as a Copilot signal.
• Updated the inline comment documenting Copilot detection inputs.
• Added a targeted telemetry test case verifying COPILOT_AGENT=1 resolves to "copilot".

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Cli/dotnet/Telemetry/LLMEnvironmentDetectorForTelemetry.cs	Adds COPILOT_AGENT to the Copilot environment detection rule and updates the rule comment accordingly.
test/dotnet.Tests/TelemetryTests/TelemetryCommonPropertiesTests.cs	Adds a new theory data case asserting COPILOT_AGENT triggers "copilot" detection.

[Youssef1313]

Should the new env variable be a boolean rule?

https://github.com/microsoft/vscode/pull/316267/changes seems to be setting it to '1'.

[marcpopMSFT]

@baronfel do we want to differentiate between the CLI and VSCode usage in our telemetry?

[baronfel]

For this field IMO no - it should attribute to copilot. For your other PR we can attribute to the 'host' (vscode, etc) and I think we should update your PR to check for this signal too.

[marcpopMSFT]

Are you thinking we'll take the other PR? That one feels more hacky and harder to justify in servicing. Hence my thought of splitting this up with the simpler env check.

[baronfel]

Yeah, I was thinking we'd polish it up and take it. Harder to justify in servicing, sure - but fine for the 11 and 10.0.4xx releases.

[dotnet-policy-service]

This PR has been labeled with Area-Telemetry. Please ensure that any telemetry changes in this PR

• comply with the published guidance * are added to the repo-local telemetry documentation * get a matching .NET Docs issue raised to document the telemetry changes
  • Go to https://learn.microsoft.com/dotnet/core/tools/telemetry
  • Scroll down to the bottom and click the 'Open a documentation issue' link to create an issue with pre-filled details