ui_input: TextField -> SingleLineInput (#28031)

- Rename `TextField` -> `SingleLineInput`
- Add a component preview for `SingleLineInput`
- Apply `SingleLineInput` to the AddContextServerModal

Release Notes:

- N/A

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Danilo Leal <67129314+danilo-leal@users.noreply.github.com>

crates/agent/Cargo.toml

@@ -81,6 +81,7 @@ theme.workspace = true
 time.workspace = true
 time_format.workspace = true
 ui.workspace = true
+ui_input.workspace = true
 util.workspace = true
 uuid.workspace = true
 vim_mode_setting.workspace = true