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/cli/doctor/checks/system-loaded-version.ts

@@ -1,7 +1,7 @@
-import { existsSync, readFileSync, realpathSync } from "node:fs"
+import { existsSync, readFileSync } from "node:fs"
 import { homedir } from "node:os"
 import { join } from "node:path"
-
+import { resolveSymlink } from "../../../shared/file-utils"
 import { getLatestVersion } from "../../../hooks/auto-update-checker/checker"
 import { extractChannel } from "../../../hooks/auto-update-checker"
 import { PACKAGE_NAME } from "../constants"
@@ -38,12 +38,7 @@ function resolveOpenCodeCacheDir(): string {
 
 function resolveExistingDir(dirPath: string): string {
   if (!existsSync(dirPath)) return dirPath
-
-  try {
-    return realpathSync(dirPath)
-  } catch {
-    return dirPath
-  }
+  return resolveSymlink(dirPath)
 }
 
 function readPackageJson(filePath: string): PackageJsonShape | null {