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/shared/opencode-config-dir.ts

@@ -1,6 +1,6 @@
 import { existsSync, realpathSync } from "node:fs"
 import { homedir } from "node:os"
-import { join, resolve } from "node:path"
+import { join, resolve, win32 } from "node:path"
 
 import type {
   OpenCodeBinaryType,
@@ -31,7 +31,7 @@ function getTauriConfigDir(identifier: string): string {
 
     case "win32": {
       const appData = process.env.APPDATA || join(homedir(), "AppData", "Roaming")
-      return join(appData, identifier)
+      return win32.join(appData, identifier)
     }
 
     case "linux":
@@ -71,7 +71,10 @@ export function getOpenCodeConfigDir(options: OpenCodeConfigDirOptions): string
   }
 
   const identifier = isDevBuild(version) ? TAURI_APP_IDENTIFIER_DEV : TAURI_APP_IDENTIFIER
-  const tauriDir = resolveConfigPath(getTauriConfigDir(identifier))
+  const tauriDirBase = getTauriConfigDir(identifier)
+  const tauriDir = process.platform === "win32"
+    ? (win32.isAbsolute(tauriDirBase) ? win32.normalize(tauriDirBase) : win32.resolve(tauriDirBase))
+    : resolveConfigPath(tauriDirBase)
 
   if (checkExisting) {
     const legacyDir = getCliConfigDir()