Support @-mentions in inline assists and when editing old agent panel messages (#29734)

Closes #ISSUE

Co-authored-by: Bennet <bennet@zed.dev>

Release Notes:

- Added support for context `@mentions` in the inline prompt editor and
when editing past messages in the agent panel.

---------

Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/eval/src/example.rs

@@ -119,6 +119,7 @@ impl ExampleContext {
                     text.to_string(),
                     ContextLoadResult::default(),
                     None,
+                    Vec::new(),
                     cx,
                 );
             })