fix: support mutable language tab configuration

src/ruby.rs

@@ -1006,7 +1006,8 @@ mod tests {
             r#"
 Vamp.configure do |c|
   c.tab_spaces = 4
-  c.language_tab_spaces = { "ruby" => 2, "makefile" => nil, "json" => 1, "yaml" => 255, "cpp" => 3, "typescriptreact" => 5 }
+  c.language_tab_spaces.merge!("ruby" => 2, "makefile" => nil, "json" => 1, "yaml" => 255, "typescriptreact" => 5)
+  c.language_tab_spaces["cpp"] = 3
 end
 "#,
         );
@@ -1069,12 +1070,18 @@ end
             for assignment in [
                 format!("c.tab_spaces = {value}"),
                 format!(r#"c.language_tab_spaces = {{ "ruby" => {value} }}"#),
+                format!(r#"c.language_tab_spaces.merge!("ruby" => {value})"#),
+                format!(r#"c.language_tab_spaces["ruby"] = {value}"#),
             ] {
                 let config = fixture.file(
                     "config.rb",
                     &format!("Vamp.configure {{ |c| {assignment} }}"),
                 );
-                assert!(load_config(&config).is_err(), "{assignment}");
+                let error = load_config(&config).unwrap_err();
+                assert!(
+                    error.contains("tab_spaces must be nil or an Integer"),
+                    "{assignment}: {error}"
+                );
             }
         }
         for value in [
@@ -1092,6 +1099,24 @@ end
         }
     }
 
+    #[test]
+    fn language_tab_space_mutations_reject_invalid_keys_before_serialization() {
+        let fixture = Fixture::new();
+        for key in [r#""""#, ":ruby", r#""\n""#, "7"] {
+            for assignment in [
+                format!("c.language_tab_spaces.merge!({key} => 2)"),
+                format!("c.language_tab_spaces[{key}] = 2"),
+            ] {
+                let config = fixture.file(
+                    "config.rb",
+                    &format!("Vamp.configure {{ |c| {assignment} }}"),
+                );
+                let error = load_config(&config).unwrap_err();
+                assert!(error.contains("Tab language"), "{assignment}: {error}");
+            }
+        }
+    }
+
     #[test]
     fn actual_ruby_dsl_context_queued_effects_and_output_isolation() {
         let fixture = Fixture::new();