fix: filter whitespace-only lines in multi-line paste

Change .filter((l) => l.length > 0) to .filter((l) => l.trim().length > 0)
so that whitespace-only lines are excluded. Previously, a paste like
"db.test\n   \n.find()" would produce a spurious double space.

Author: tnaum-ms

src/documentdb/shell/DocumentDBShellPty.ts

@@ -267,7 +267,7 @@ export class DocumentDBShellPty implements vscode.Pseudoterminal {
      */
     private async handleMultiLinePaste(data: string): Promise<void> {
         // Normalize line endings and split
-        const lines = data.split(/\r\n|\r|\n/).filter((l) => l.length > 0);
+        const lines = data.split(/\r\n|\r|\n/).filter((l) => l.trim().length > 0);
 
         // If only one non-empty line after splitting, process normally
         if (lines.length <= 1) {