PY-85822 remove @RequiresBackgroundThread from PythonPackageManager.forSdk()

The annotation was guarding the old blocking implementation that called
runBlockingMaybeCancellable unconditionally. Since PY-87287, that call
is gated behind shouldBeInitInstantly() (true only in unit-test mode),
so forSdk() no longer blocks in production.

On cache hit the method is a plain ConcurrentHashMap lookup. On cache
miss it creates the manager, registers Disposer listeners, and sets up
VFS watchers — lightweight work that in practice runs on a background
thread during project/SDK setup, so subsequent EDT callers always get
a cached instance.

The stale annotation caused ISE on EDT, killing the entire Run action
(ExecutorAction.runCurrentFile → PyTestsConfigurationProducer →
isFrameworkInstalled → PythonPackageManager.forSdk).


(cherry picked from commit f2dae4a6e2e3b2915e92641007302e7e641eae3e)

IJ-MR-194254

GitOrigin-RevId: f197757763ca50acc9e927a82a79e75ff8151889

python/src/com/jetbrains/python/packaging/management/PythonPackageManager.kt

@@ -346,7 +346,6 @@ abstract class PythonPackageManager(val project: Project, val sdk: Sdk) : Dispos
   companion object {
     private val CACHE_KEY = Key.create<CachedValue<Deferred<PyResult<List<PythonPackage>>?>>>("PythonPackageManagerDependenciesCache")
 
-    @RequiresBackgroundThread
     @Throws(AlreadyDisposedException::class)
     fun forSdk(project: Project, sdk: Sdk): PythonPackageManager {
       val pythonPackageManagerService = project.service<PythonPackageManagerService>()

python/src/com/jetbrains/python/packaging/management/PythonPackageManagerServiceImpl.kt

@@ -24,7 +24,16 @@ internal class PythonPackageManagerServiceImpl(private val serviceScope: Corouti
   private val bridgeCache = ConcurrentHashMap<UUID, PythonPackageManagementServiceBridge>()
 
   /**
-   * Requires Sdk to be Python Sdk and have PythonSdkAdditionalData.
+   * Returns a cached [PythonPackageManager] for the given [sdk], creating one on first access.
+   *
+   * On cache hit the call is effectively free (a [ConcurrentHashMap] lookup).
+   * On cache miss (once per SDK per project lifetime) the method creates the manager,
+   * registers Disposer listeners and sets up VFS watchers — this may involve lightweight I/O
+   * (e.g. flavor detection in [com.jetbrains.python.sdk.getOrCreateAdditionalData]).
+   * In practice the first call happens during project/SDK setup on a background thread,
+   * so subsequent EDT callers always get a cached instance.
+   *
+   * Requires [sdk] to be a Python SDK with [com.jetbrains.python.sdk.PythonSdkAdditionalData].
    */
   override fun forSdk(project: Project, sdk: Sdk): PythonPackageManager {
     val cacheKey = (sdk.getOrCreateAdditionalData()).uuid