sessions: support signed-out Copilot Agent Host sessions

[Vritant Bhardwaj (vritant24)]

Summary

• Guides signed-out Copilot Agent Host users to configure a BYOK model when none is ready.
• Allows sessions using an advertised BYOK model to start without GitHub authentication.
• Keeps GitHub authentication required for models and agents that still depend on it.

Technical context for AI-assisted review

Intent and previous behavior

The signed-out Agent Host gate from the base branch makes the Copilot Agent Host target available when BYOK use is enabled. Session creation still treated every selected model as requiring the agent's GitHub authentication, however, and users without a ready model had no guidance in the chat input.

Implementation

A session-type-scoped chat input notification observes sign-in state, extension registration, configuration, and Agent Host model availability. It offers model management and sign-in actions when the Copilot Agent Host target is selected but no usable BYOK model is available.

The session handler now passes the selected model into the authentication decision. Required agent authentication is bypassed only when that model is an advertised BYOK model; all other selections continue through the existing authentication flow.

Behavior and constraints

• The notification applies only to the Copilot Agent Host session type while signed out and while signed-out use is enabled.
• Model discovery must settle before the notification decides that setup is required.
• A model's advertised BYOK status, rather than the setting alone, controls the authentication bypass.
• Existing authentication behavior remains unchanged for GitHub-backed models and other session providers.

Review context

This change is stacked on #330082 and relies on its signed-out session-type availability behavior.

[Copilot]

Pull request overview

Adds signed-out BYOK support for Copilot Agent Host sessions while retaining authentication for GitHub-backed models.

Changes:

• Adds signed-out model setup guidance.
• Makes authentication depend on the selected model’s advertised BYOK metadata.
• Adds related availability and authentication tests.

Show a summary per file

File	Description
agentHostAuth.test.ts	Tests model-specific authentication decisions.
agentHostSignedOutModelsNotification.ts	Adds signed-out model guidance.
agentHostSessionHandler.ts	Passes model selection into authentication.
agentHostAuth.ts	Implements BYOK authentication bypass.
sessionTypeAuthRequirement.test.ts	Tests notification readiness and visibility.
localAgentHost.contribution.ts	Registers the notification contribution.

Review details

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• Files reviewed: 6/6 changed files
• Comments generated: 2
• Review effort level: Balanced

The base branch was changed.

The base branch was changed.