acp: Fix @mentions when remoting from Windows to Linux (#38882)

Closes #38620

`Url::from_file_path` and `Url::from_directory_path` assume the path
style of the target they were compiled for, so we can't use them in
general. So, switch from `file://` to encoding the absolute path (for
mentions that have one) as a query parameter, which works no matter the
platforms. We'll still parse the old `file://` mention URIs for
compatibility with thread history.

Release Notes:

- windows: Fixed a crash when using `@mentions` in agent threads when
remoting from Windows to Linux or WSL.

crates/agent2/src/agent.rs

@@ -1205,7 +1205,7 @@ mod tests {
     use acp_thread::{AgentConnection, AgentModelGroupName, AgentModelInfo, MentionUri};
     use fs::FakeFs;
     use gpui::TestAppContext;
-    use indoc::indoc;
+    use indoc::formatdoc;
     use language_model::fake_provider::FakeLanguageModel;
     use serde_json::json;
     use settings::SettingsStore;
@@ -1502,13 +1502,17 @@ mod tests {
         summary_model.end_last_completion_stream();
 
         send.await.unwrap();
+        let uri = MentionUri::File {
+            abs_path: path!("/a/b.md").into(),
+        }
+        .to_uri();
         acp_thread.read_with(cx, |thread, cx| {
             assert_eq!(
                 thread.to_markdown(cx),
-                indoc! {"
+                formatdoc! {"
                     ## User
 
-                    What does [@b.md](file:///a/b.md) mean?
+                    What does [@b.md]({uri}) mean?
 
                     ## Assistant
 
@@ -1544,10 +1548,10 @@ mod tests {
         acp_thread.read_with(cx, |thread, cx| {
             assert_eq!(
                 thread.to_markdown(cx),
-                indoc! {"
+                formatdoc! {"
                     ## User
 
-                    What does [@b.md](file:///a/b.md) mean?
+                    What does [@b.md]({uri}) mean?
 
                     ## Assistant