fix(tests): resolve 6 test isolation failures in full suite

3 failures in fallback.cliproxyapi-matrix.test.ts:
- Root cause: leaked spyOn(getMainSessionID) in
  tool-execute-before-session-notification.test.ts was never restored,
  poisoning module state for subsequent tests in the same worker
- Added mockRestore() call and _resetModelFallbackForTesting in afterEach

3 failures in background-agent/manager.test.ts:
- Root cause: connected-providers-cache memConnected/memProviderModels
  persisted across test files, making isReachable() skip fallback
  candidates in retry tests
- Added mock.module for connected-providers-cache at file level
- Added _resetMemCacheForTesting export and global beforeEach cleanup

Full suite: 4638/4638 pass, 0 fail

src/plugin/tool-execute-before-session-notification.test.ts

@@ -27,6 +27,8 @@ describe("createToolExecuteBeforeHandler session notification sessionID", () =>
 
     expect(getMainSessionIDSpy).toHaveBeenCalled()
     expect(capturedSessionID).toBe(mainSessionID)
+
+    getMainSessionIDSpy.mockRestore()
   })
 })