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/features/background-agent/fallback-retry-handler.test.ts

@@ -1,4 +1,4 @@
-import { describe, test, expect, mock, beforeEach } from "bun:test"
+import { afterAll, beforeEach, describe, expect, mock, test } from "bun:test"
 
 mock.module("../../shared", () => ({
   log: mock(() => {}),
@@ -82,6 +82,10 @@ function createDefaultArgs(taskOverrides: Partial<BackgroundTask> = {}) {
 }
 
 describe("tryFallbackRetry", () => {
+  afterAll(() => {
+    mock.restore()
+  })
+
   beforeEach(() => {
     ;(shouldRetryError as any).mockImplementation(() => true)
     ;(selectFallbackProvider as any).mockImplementation((providers: string[]) => providers[0])
@@ -274,8 +278,8 @@ describe("tryFallbackRetry", () => {
 
   describe("#given disconnected fallback providers with connected preferred provider", () => {
     test("keeps fallback entry and selects connected preferred provider", () => {
-      ;(readProviderModelsCache as any).mockReturnValue({ connected: ["provider-a"] })
-      ;(selectFallbackProvider as any).mockImplementation(
+      ;(readProviderModelsCache as any).mockReturnValueOnce({ connected: ["provider-a"] })
+      ;(selectFallbackProvider as any).mockImplementationOnce(
         (_providers: string[], preferredProviderID?: string) => preferredProviderID ?? "provider-b",
       )