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/agent/src/tests/mod.rs

@@ -2553,7 +2553,7 @@ fn setup_context_server(
         let mut settings = ProjectSettings::get_global(cx).clone();
         settings.context_servers.insert(
             name.into(),
-            project::project_settings::ContextServerSettings::Custom {
+            project::project_settings::ContextServerSettings::Stdio {
                 enabled: true,
                 command: ContextServerCommand {
                     path: "somebinary".into(),