Switch from deprecated serde_yaml to serde_yaml_ng

serde_yaml is deprecated and unmaintained. serde_yaml_ng is the community
fork with a compatible API, so this is a straightforward rename across the
codebase.

src/controller/agent/create/tests.rs

@@ -2,12 +2,12 @@
 fn agent_config_parsing_works() {
     struct TestCase {
         input: String,
-        expected: Option<serde_yaml::Value>,
+        expected: Option<serde_yaml_ng::Value>,
     }
 
     let provider = crate::agent::AgentProvider::OpenAI;
     let sample_config = crate::agent::default_config_for_provider(&provider);
-    let sample_config_pretty_yaml = serde_yaml::to_string(&sample_config).unwrap();
+    let sample_config_pretty_yaml = serde_yaml_ng::to_string(&sample_config).unwrap();
 
     let test_cases = vec![
         // Invalid input