test: add single-entry multiline inline table insertion test

Adds an explicit test reproducing the issue from PR #5655 comment
where inserting into a single-entry multiline inline table without a
trailing comma could produce malformatted output. The fix already
handles this case correctly.

https://claude.ai/code/session_01FT59TTz4yRy3DPwbREE82u

Author: claude

crates/pixi_manifest/src/toml/document.rs

@@ -489,4 +489,34 @@ dependencies = {
         }
         "#);
     }
+
+    /// Reproduces the bug from https://github.com/prefix-dev/pixi/pull/5655#issuecomment-4040362659
+    /// where inserting into a single-entry multiline inline table without a
+    /// trailing comma produces `{ numpy = "*"\n, pydantic = ... }`.
+    #[test]
+    fn test_insert_into_multiline_inline_table_single_entry() {
+        let toml = r#"
+[feature.test]
+dependencies = {
+    numpy = "*"
+}
+"#;
+        let mut manifest = TomlDocument::new(DocumentMut::from_str(toml).unwrap());
+        manifest
+            .insert_into_inline_table(
+                &["feature", "test", "dependencies"],
+                "pydantic",
+                toml_edit::Value::from(">=2.12.5,<3"),
+            )
+            .unwrap();
+
+        insta::assert_snapshot!(manifest.0.to_string(), @r#"
+
+        [feature.test]
+        dependencies = {
+            numpy = "*",
+            pydantic = ">=2.12.5,<3"
+        }
+        "#);
+    }
 }