ai: Auto select user model when there's no default (#36722)

This PR identifies automatic configuration options that users can select
from the agent panel. If no default provider is set in their settings,
the PR defaults to the first recommended option. Additionally, it
updates the selected provider for a thread when a user changes the
default provider through the settings file, if the thread hasn't had any
queries yet.

Release Notes:

- agent: automatically select a language model provider if there's no
user set provider.

---------

Co-authored-by: Michael Sloan <michael@zed.dev>

crates/git_ui/src/git_panel.rs

@@ -4466,7 +4466,7 @@ fn current_language_model(cx: &Context<'_, GitPanel>) -> Option<Arc<dyn Language
     is_enabled
         .then(|| {
             let ConfiguredModel { provider, model } =
-                LanguageModelRegistry::read_global(cx).commit_message_model()?;
+                LanguageModelRegistry::read_global(cx).commit_message_model(cx)?;
 
             provider.is_authenticated(cx).then(|| model)
         })