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.test.ts

@@ -1,9 +1,10 @@
 import { afterEach, describe, expect, it } from "bun:test"
-import { mkdirSync, mkdtempSync, realpathSync, rmSync, symlinkSync, writeFileSync } from "node:fs"
+import { mkdirSync, mkdtempSync, rmSync, symlinkSync, writeFileSync } from "node:fs"
 import { tmpdir } from "node:os"
 import { dirname, join } from "node:path"
 
 import { PACKAGE_NAME } from "../constants"
+import { resolveSymlink } from "../../../shared/file-utils"
 
 const systemLoadedVersionModulePath = "./system-loaded-version?system-loaded-version-test"
 
@@ -125,7 +126,7 @@ describe("system loaded version", () => {
       const loadedVersion = getLoadedPluginVersion()
 
       //#then
-      expect(loadedVersion.cacheDir).toBe(realpathSync(symlinkConfigDir))
+      expect(loadedVersion.cacheDir).toBe(resolveSymlink(symlinkConfigDir))
       expect(loadedVersion.expectedVersion).toBe("4.5.6")
       expect(loadedVersion.loadedVersion).toBe("4.5.6")
     })