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.

test-setup.ts

@@ -1,6 +1,8 @@
 import { beforeEach } from "bun:test"
-import { _resetForTesting } from "./src/features/claude-code-session-state/state"
+import { _resetForTesting as resetClaudeSessionState } from "./src/features/claude-code-session-state/state"
+import { _resetForTesting as resetModelFallbackState } from "./src/hooks/model-fallback/hook"
 
 beforeEach(() => {
-  _resetForTesting()
+  resetClaudeSessionState()
+  resetModelFallbackState()
 })