Return 409 PathAlreadyExists when creating an existing directory via DFS

Azure ADLS Gen2 returns 409 when creating a directory that already exists.
The SDK's CreateIfNotExistsAsync relies on this to return null/false for
existing directories. Azurite was returning 201 regardless, causing
CreateDirectory_AlreadyExists_ReturnsFalse to fail.

Now checks if a directory blob with hdi_isfolder=true already exists
before creating, and returns 409 PathAlreadyExists if so. File creates
remain idempotent (overwrite) matching Azure behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Skrypt

src/blob/dfs/handlers/PathHandler.ts

@@ -11,6 +11,7 @@ import { getDfsContext, IDfsContext } from "../DfsContext";
 import {
   sendDfsError,
   pathNotFound,
+  pathAlreadyExists,
   filesystemNotFound,
   directoryNotEmpty,
   internalError,
@@ -53,6 +54,16 @@ export default class PathHandler {
         metadata[HNS_DIRECTORY_METADATA_KEY] = "true";
       }
 
+      // Azure returns 409 PathAlreadyExists when creating a directory that
+      // already exists. This is required for the SDK's CreateIfNotExistsAsync
+      // to correctly return null for existing directories.
+      if (isDirectory) {
+        const existing = await this.safeGetBlobProperties(account, filesystem, pathName);
+        if (existing && existing.metadata?.[HNS_DIRECTORY_METADATA_KEY] === "true") {
+          return sendDfsError(res, pathAlreadyExists(pathName));
+        }
+      }
+
       // Ensure intermediate directories exist
       if (pathName.includes("/")) {
         await this.ensureIntermediateDirectories(account, filesystem, pathName, now);