[SPARK-52289] Enable jsonToDdl test in Linux environment

### What changes were proposed in this pull request?

This PR aims to enable `jsonToDdl` test in Linux environment.

### Why are the changes needed?

Previous, the test case was disabled in Linux environment because `JsonToDDL` message was added after Apache Spark 4.0.0-preview2.
- apache/spark#49142

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Manual test because the Linux CI is moved to the daily CI.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #172 from dongjoon-hyun/SPARK-52289.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

Tests/SparkConnectTests/SparkConnectClientTests.swift

@@ -84,7 +84,6 @@ struct SparkConnectClientTests {
     await client.stop()
   }
 
-#if !os(Linux)  // TODO: Enable this with the offical Spark 4 docker image
   @Test
   func jsonToDdl() async throws {
     let client = SparkConnectClient(remote: TEST_REMOTE)
@@ -96,5 +95,4 @@ struct SparkConnectClientTests {
     }
     await client.stop()
   }
-#endif
 }