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.test.ts

@@ -1,6 +1,6 @@
 import { describe, test, expect, beforeEach, afterEach } from "bun:test"
 import { homedir } from "node:os"
-import { join, resolve } from "node:path"
+import { join, resolve, win32 } from "node:path"
 import {
   getOpenCodeConfigDir,
   getOpenCodeConfigPaths,
@@ -241,9 +241,10 @@ describe("opencode-config-dir", () => {
         // when getOpenCodeConfigDir is called with binary="opencode-desktop"
         const result = getOpenCodeConfigDir({ binary: "opencode-desktop", version: "1.0.200", checkExisting: false })
 
-        // then returns %APPDATA%/ai.opencode.desktop
-        expect(result).toBe(join("C:\\Users\\TestUser\\AppData\\Roaming", TAURI_APP_IDENTIFIER))
+        // then returns %APPDATA%/ai.opencode.desktop using Windows path semantics
+        expect(result).toBe(win32.join("C:\\Users\\TestUser\\AppData\\Roaming", TAURI_APP_IDENTIFIER))
       })
+
     })
 
     describe("dev build detection", () => {