Add provider list to the Configure LLM Providers modal

[bricestacey]

Fixes #14817

Summary

Renders the grouped provider list that is the main content of the new "Configure LLM Providers" modal (epic #14815), following the provider-configuration design spec (IMPLEMENTATION_SPEC.md / the assistant-providers prototype). The list is driven off the IPositronLanguageModelSource[] the modal already receives.

Sections (alphabetical within each, empty built-in sections hidden):

• Connected Providers - signed in and healthy; shows an auth badge (Environment / OAuth) and an "Edit" action.
• Needs Attention - signed in but reporting an error; shows an "Error" badge, the error message, and a "Fix Connection" action.
• Model Providers - not yet connected; shows a one-line description and a "+ Connect" action.
• Custom Provider - always present (Experimental badge + "+ Add custom provider" button) for OpenAI-compatible endpoints.

Each row shows a rounded-square provider avatar (36x36, 6px radius), the provider name, and a maturity badge (Preview / Experimental; none for GA). The per-section action button is the only interactive element - the row itself is not selectable - and the buttons are no-ops for now so the connect flow (#14818) and connected-provider details (#14819) can hang off them later.

Notes on data:

• Positron provider metadata has no description field yet, so the "Model Providers" descriptions come from a small hardcoded id->string map mirroring the prototype copy (missing ids render no description).
• Auth badges are derived from defaults.autoconfigure (Environment) and supportedOptions (OAuth).
• Admin-managed mode (banner, Managed badge, visibleProviders) is deferred - Positron has no admin-managed data source wired yet.
• Per-provider avatar background colors from the design are not applied yet; the avatar uses a neutral themed square (open question - see the issue).

Behind the hidden assistant.newProviderModal feature switch (default off), so no user-visible impact until enabled.

Stacked on #14856 (adds the modal shell + feature switch). Based on that branch; retargets to main once #14856 merges.

Release Notes

New Features

• N/A (in-progress feature behind the hidden assistant.newProviderModal switch)

Bug Fixes

• N/A

Validation Steps

@:assistant

Covered by unit tests:

npx vitest run \
  src/vs/workbench/contrib/positronAssistant/test/common/providerGrouping.vitest.ts \
  src/vs/workbench/contrib/positronAssistant/test/browser/providerListItem.vitest.tsx \
  src/vs/workbench/contrib/positronAssistant/test/browser/providerList.vitest.tsx

Manual:

1. Set "assistant.newProviderModal": true in settings.json.
2. Open the language model provider configuration (same entry point as the legacy dialog).
3. Verify providers appear grouped under Connected Providers / Needs Attention / Model Providers with the right badges, descriptions, rounded-square avatars, and action buttons (Connect/Add custom provider show a plus icon); the Custom Provider section is always present; empty built-in sections are absent.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:assistant

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Tag	Source
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@:assistant	PR description

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[bricestacey]

:smh: Claude Code is opening PRs willy nilly