mcp: Fix source property showing up as undefined in settings (#43417)

Follow up to #39021.

<img width="576" height="141" alt="image"
src="https://github.com/user-attachments/assets/c89885a4-e664-4614-9bb0-86442dff34ee"
/>

- Add migration to remove `source` tag because `ContextServerSettings`
is now untagged
- Fix typos in context server modal
- PR seems to have removed the `test_action_namespaces` test, which I
brought back in this PR

Release Notes:

- Fixed an issue where the `source` property of MCP settings would show
up as unrecognised

crates/settings/src/settings_content/project.rs

@@ -192,7 +192,7 @@ pub struct SessionSettingsContent {
 #[derive(Deserialize, Serialize, Clone, PartialEq, Eq, JsonSchema, MergeFrom, Debug)]
 #[serde(untagged, rename_all = "snake_case")]
 pub enum ContextServerSettingsContent {
-    Custom {
+    Stdio {
         /// Whether the context server is enabled.
         #[serde(default = "default_true")]
         enabled: bool,
@@ -225,7 +225,7 @@ pub enum ContextServerSettingsContent {
 impl ContextServerSettingsContent {
     pub fn set_enabled(&mut self, enabled: bool) {
         match self {
-            ContextServerSettingsContent::Custom {
+            ContextServerSettingsContent::Stdio {
                 enabled: custom_enabled,
                 ..
             } => {

crates/settings/src/vscode_import.rs

@@ -568,7 +568,7 @@ impl VsCodeSettings {
             .filter_map(|(k, v)| {
                 Some((
                     k.clone().into(),
-                    ContextServerSettingsContent::Custom {
+                    ContextServerSettingsContent::Stdio {
                         enabled: true,
                         command: serde_json::from_value::<VsCodeContextServerCommand>(v.clone())
                             .ok()