elixir: Make two more files required by lexical executable (#16382)

I still haven't fixed building dev extensions with rust managed by nix,
so I'd appreciate testing this for me

Release Notes:

- N/A

assets/settings/default.json

@@ -395,9 +395,9 @@
     // The default model to use when creating new contexts.
     "default_model": {
       // The provider to use.
-      "provider": "openai",
+      "provider": "zed.dev",
       // The model to use.
-      "model": "gpt-4o"
+      "model": "claude-3-5-sonnet"
     }
   },
   // The settings for slash commands.

crates/assistant/src/assistant_settings.rs

@@ -543,8 +543,8 @@ mod tests {
             assert_eq!(
                 AssistantSettings::get_global(cx).default_model,
                 LanguageModelSelection {
-                    provider: "openai".into(),
-                    model: "gpt-4o".into(),
+                    provider: "zed.dev".into(),
+                    model: "claude-3-5-sonnet".into(),
                 }
             );
         });

crates/zed/RELEASE_CHANNEL

@@ -1 +1 @@
-dev
+preview

extensions/elixir/src/language_servers/lexical.rs

@@ -95,6 +95,8 @@ impl Lexical {
             .map_err(|e| format!("failed to download file: {e}"))?;
 
             zed::make_file_executable(&binary_path)?;
+            zed::make_file_executable(&format!("{version_dir}/lexical/bin/debug_shell.sh"))?;
+            zed::make_file_executable(&format!("{version_dir}/lexical/priv/port_wrapper.sh"))?;
 
             let entries =
                 fs::read_dir(".").map_err(|e| format!("failed to list working directory {e}"))?;