agent: Add design adjustments to MCP config flow (#29765)

Mostly somewhat small UI tweaks around the MCP extension config flow and
the settings section.

Release Notes:

- N/A

crates/icons/src/icons.rs

@@ -137,6 +137,7 @@ pub enum IconName {
     GitBranchSmall,
     Github,
     Globe,
+    Hammer,
     Hash,
     HistoryRerun,
     Image,