fix: Provider.models is a Record keyed by modelID, not an array

Per the OpenCode server docs (GET /config/providers), each Provider's
'models' is { [modelID: string]: Model } (a Record), so model IDs can be
referenced as strings (matching the 'default' map of providerID ->
modelID). Our code treated it as Model[], which:
  1. crashed with 'p.models is not iterable' on real responses, and
  2. after the previous Array.isArray guard, skipped EVERY provider
     (a Record is never an array), so the inline picker showed nothing.

Update types.ts (OpenCodeProvider.models -> Record, add OpenCodeModel,
add optional 'default' map to the response) and rewrite
flattenProviders to iterate Object.keys(p.models). Update test fixture
to the Record shape.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/__tests__/opencode_cell_actions.spec.ts

@@ -184,8 +184,13 @@ describe('OpenCodeCellActions', () => {
   it('renders a model selector with the first option selected when providers are cached', () => {
     setOpenCodeProviders({
       providers: [
-        { id: 'anthropic', models: [{ id: 'claude-sonnet-4-20250514' }] },
-        { id: 'openai', models: [{ id: 'gpt-5' }] }
+        {
+          id: 'anthropic',
+          models: {
+            'claude-sonnet-4-20250514': { id: 'claude-sonnet-4-20250514' }
+          }
+        },
+        { id: 'openai', models: { 'gpt-5': { id: 'gpt-5' } } }
       ]
     });
     const cell = makeFakeCell('x = 1');