fix(tests): resolve 5 cross-file test isolation failures

- model-fallback hook: mock selectFallbackProvider and add _resetForTesting()
  to test-setup.ts to clear module-level state between files
- fallback-retry-handler: add afterAll(mock.restore) and use mockReturnValueOnce
  to prevent connected-providers mock leaking to subsequent test files
- opencode-config-dir: use win32.join for Windows APPDATA path construction
  so tests pass on macOS (path.join uses POSIX semantics regardless of
  process.platform override)
- system-loaded-version: use resolveSymlink from file-utils instead of
  realpathSync to handle macOS /var -> /private/var symlink consistently

All 4456 tests pass (0 failures) on full bun test suite.

src/hooks/model-fallback/hook.ts

@@ -274,3 +274,13 @@ export function createModelFallbackHook(args?: { toast?: FallbackToast; onApplie
     },
   }
 }
+
+/**
+ * Resets all module-global state for testing.
+ * Clears pending fallbacks, toast keys, and session chains.
+ */
+export function _resetForTesting(): void {
+  pendingModelFallbacks.clear()
+  lastToastKey.clear()
+  sessionFallbackChains.clear()
+}