Improve error handling

[chrmarti]

microsoft/vscode#290234

[Copilot]

Pull request overview

Improves Copilot Chat authentication error handling and related UX, including a dedicated state for GitHub login failures.

Changes:

• Add a debug configuration switch to simulate specific GitHub auth/token failure reasons.
• Introduce a distinct GitHubLoginFailed error type and surface it via a new welcome message/context key.
• Add additional logging around activation/deactivation and context key updates.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/platform/configuration/common/configurationService.ts	Adds a new debug setting for simulating GitHub auth failures.
src/platform/authentication/vscode-node/copilotTokenManager.ts	Adds GitHubLoginFailedError and supports debug-simulated auth failure reasons.
src/extension/conversation/vscode-node/conversationFeature.ts	Adds log statements when activating/deactivating contributions.
src/extension/contextKeys/vscode-node/contextKeys.contribution.ts	Adds a new context key/state for GitHub login failure and updates error→context mapping.
package.nls.json	Updates the invalid-token welcome message action and adds a new GitHub-login-failed welcome message.
package.json	Registers the new welcome card and updates walkthrough gating conditions to include the new state.

[Tyriar]

@chrmarti consider the comments above ☝️