Fix type-checking errors in snippets (code only, no .md)

This commit both:
- Fixes pre-existing mistakes in snippets, aka wrong code samples.
- Adds hidden (`@hide`) statements that do not appear in the docs,
  but satisfy typecheckers.

qdrant-landing/content/documentation/headless/snippets/check-collection-exists/simple/csharp.cs

@@ -1,9 +1,10 @@
-
+using Qdrant.Client; // @hide
 
 public class Snippet
 {
 	public static async Task Run()
 	{
+		var client = new QdrantClient("localhost", 6334); // @hide
 		await client.CollectionExistsAsync("{collection_name}");
 	}
 }

qdrant-landing/content/documentation/headless/snippets/check-collection-exists/simple/go.go

@@ -1,7 +1,16 @@
 package snippet
 
 import "context"
+import "github.com/qdrant/go-client/qdrant" // @hide
 
 func Main() {
+	// @hide-start
+	client, err := qdrant.NewClient(&qdrant.Config{
+	    Host: "localhost",
+	    Port: 6334,
+	})
+	if err != nil { panic(err) }
+	// @hide-end
+
 	client.CollectionExists(context.Background(), "my_collection")
 }

qdrant-landing/content/documentation/headless/snippets/check-collection-exists/simple/java.java

@@ -2,6 +2,10 @@ package com.example.snippets_amalgamation;
 
 public class Snippet {
         public static void run() throws Exception {
+                // @hide-start
+                io.qdrant.client.QdrantClient client =
+                    new io.qdrant.client.QdrantClient(io.qdrant.client.QdrantGrpcClient.newBuilder("localhost", 6334, false).build());
+                // @hide-end
                 client.collectionExistsAsync("{collection_name}").get();
         }
 }

qdrant-landing/content/documentation/headless/snippets/check-collection-exists/simple/rust.rs

@@ -1,6 +1,8 @@
 
 
 pub async fn main() -> anyhow::Result<()> {
+    let client = qdrant_client::Qdrant::from_url("http://localhost:6334").build()?; // @hide
+
     client.collection_exists("{collection_name}").await?;
 
     Ok(())