agent: Rework context server settings (#32793)

This changes the way context servers are organised. We now store a
`source` which indicates if the MCP server is configured manually or
managed by an extension.

Release Notes:

- N/A

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/migrator/src/migrations.rs

@@ -75,3 +75,9 @@ pub(crate) mod m_2025_05_29 {
 
     pub(crate) use settings::SETTINGS_PATTERNS;
 }
+
+pub(crate) mod m_2025_06_16 {
+    mod settings;
+
+    pub(crate) use settings::SETTINGS_PATTERNS;
+}