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/extension_host/src/wasm_host/wit/since_v0_6_0.rs

@@ -972,7 +972,7 @@ impl ExtensionImports for WasmState {
                             });
 
                         match settings {
-                            project::project_settings::ContextServerSettings::Custom {
+                            project::project_settings::ContextServerSettings::Stdio {
                                 enabled: _,
                                 command,
                             } => Ok(serde_json::to_string(&settings::ContextServerSettings {