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/shared/connected-providers-cache.ts

@@ -222,6 +222,11 @@ export function createConnectedProvidersCacheStore(
 		}
 	}
 
+	function _resetMemCacheForTesting(): void {
+		memConnected = undefined
+		memProviderModels = undefined
+	}
+
 	return {
 		readConnectedProvidersCache,
 		hasConnectedProvidersCache,
@@ -229,6 +234,7 @@ export function createConnectedProvidersCacheStore(
 		hasProviderModelsCache,
 		writeProviderModelsCache,
 		updateConnectedProvidersCache,
+		_resetMemCacheForTesting,
 	}
 }
 
@@ -269,4 +275,5 @@ export const {
 	hasProviderModelsCache,
 	writeProviderModelsCache,
 	updateConnectedProvidersCache,
+	_resetMemCacheForTesting,
 } = defaultConnectedProvidersCacheStore