Refresh stale provider and model TUI states

[Aaronontheweb]

Summary

• Refresh the provider OAuth page after each visible state change.
• Show the authorization URL and user code after the request starts.
• Load model roles after the configuration becomes available.
• Replace the model list when the user selects a different provider.
• Add headless tests for each stale-screen defect.
• Extend the native model-manager tape with a provider switch.

Root cause

PR #1804 added keyed layouts to preserve stateful controls.

The provider OAuth key ignored visible OAuth state revisions. The layout reused the original start-screen child.

The model page created its RoleOverview child before its view model loaded the configuration. The state did not change after the load.

The model picker entered DiscoverModels before it cleared the prior probe result. The layout cached the prior provider's DiscoverList child.

The new probe updated the data under the same key. The keyed layout correctly retained its existing child, which contained stale models.

Validation

• Provider-manager tests passed 55 tests.
• Model-manager tests passed 24 tests.
• The solution build passed with zero warnings and zero errors.
• dotnet slopwatch analyze passed.
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify passed.
• git diff --check passed.
• ./scripts/smoke/run-smoke.sh light passed 23 tapes and 9 scenarios.
• The model-manager tape showed both configured roles.
• The tape changed the selected provider from alpha-models to bravo-models.
• A local binary swap passed the runtime health checks.

Fixes #1825
Fixes #1829