[air] IJAI-1 Support optional backend completion for frontend only documents/editors

This is intentioanlly implemented with the source of truth in frontend to be able to work without backend modules enabled in light mode.

The idea:
- if backend available, bind a mock backend document/editor to the existing frontend native document/editor and run completion session in the bound doc to have backend variants provided
- in light mode, do not bind because the corresponding rpc is not available - have reduced functionality and not fail/break the entire plugin

(cherry picked from commit d565129ebc9deae4facb78bff3957f075d7686f3)

GitOrigin-RevId: 15d483a87fa58b924a11be8309de950091996387

platform/platform-impl/rpc/src/com/intellij/ide/rpc/BackendBasedDocument.kt

@@ -35,11 +35,14 @@ fun Document.bindToBackend(
   service<BackendBasedDocumentCoroutineScopeProvider>().cs.launch(Dispatchers.EDT) {
     val builder = BackendDocumentBindBuilder().apply(builder)
     val backendDocumentIdProvider = builder.backendDocumentIdProvider
-    if (backendDocumentIdProvider != null) {
+    val documentBound = if (backendDocumentIdProvider != null) {
       bindToBackend(backendDocumentIdProvider, builder.onBindingDispose)
     }
+    else {
+      true
+    }
 
-    if (builder.bindEditors) {
+    if (documentBound && builder.bindEditors) {
       // mark the document, so future editors will be bind
       bindEditorsToBackend()
       // bind current editors (since they might be created during backends' documents initialization)
@@ -87,7 +90,7 @@ private fun Document.bindCurrentEditors() {
 private suspend fun Document.bindToBackend(
   backendDocumentIdProvider: suspend (FrontendDocumentId) -> BackendDocumentId?,
   onBindingDispose: (() -> Unit)?,
-) {
+): Boolean {
   val frontendDocument = this
   val frontendDocumentId = FrontendDocumentId(UID.random())
   val registry = FrontendDocumentIdRegistry.EP_NAME.extensionList.firstOrNull()
@@ -102,6 +105,7 @@ private suspend fun Document.bindToBackend(
       registry?.unregisterFrontendDocumentId(frontendDocumentId)
     }
   }
+  return backendDocumentId != null
 }
 
 @ApiStatus.Internal
@@ -188,4 +192,4 @@ interface BackendDocumentBinder {
 
 // TODO: should BackendBasedDocument take cs as argument? Let's see by usages
 @Service(Service.Level.APP)
-private class BackendBasedDocumentCoroutineScopeProvider(val cs: CoroutineScope)
+private class BackendBasedDocumentCoroutineScopeProvider(val cs: CoroutineScope)