fix: guard flattenProviders against non-array p.models

Some OpenCode Serve /config/providers responses include providers
without a 'models' field (or with a non-array value). The un-guarded
for-of on p.models then throws 'p.models is not iterable' and crashes
the inline prompt construction. Skip such providers instead.

Also re-assert the button label as 'AI' (matches the df2f3df label
simplification, which the 02cedac revert had bundled away).

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

src/__tests__/opencode_cell_actions.spec.ts

@@ -127,7 +127,7 @@ describe('OpenCodeCellActions', () => {
     const actions = new OpenCodeCellActions(cell);
     const btns = actions.node.querySelectorAll('button');
     expect(btns.length).toBe(1);
-    expect(btns[0].textContent).toContain('🪄');
+    expect(btns[0].textContent).toContain('AI');
   });
 
   it('disables the button when notebook panel cannot be resolved', () => {