Dedupe AssistantSettings (#23190)

This PR dedupes the `AssistantSettings` so we can use the same settings
for both Assistant1 and Assistant2.

We originally forked them so we could change the Assistant2 settings
freely, but given our rollout strategy for the new Assistant, I don't
think that makes sense.

This also fixes the issue where the JSON language server would show a
"Matches multiple schemas when only one must validate" warning in
`settings.json`.

Closes #23171.

Release Notes:

- Fixed the "Matches multiple schemas when only one must validate"
warning for the `assistant` setting.

crates/assistant2/src/assistant.rs

@@ -1,7 +1,6 @@
 mod active_thread;
 mod assistant_model_selector;
 mod assistant_panel;
-mod assistant_settings;
 mod buffer_codegen;
 mod context;
 mod context_picker;
@@ -21,6 +20,7 @@ mod ui;
 
 use std::sync::Arc;
 
+use assistant_settings::AssistantSettings;
 use client::Client;
 use command_palette_hooks::CommandPaletteFilter;
 use feature_flags::{Assistant2FeatureFlag, FeatureFlagAppExt};
@@ -31,7 +31,6 @@ use settings::Settings as _;
 use util::ResultExt;
 
 pub use crate::assistant_panel::AssistantPanel;
-use crate::assistant_settings::AssistantSettings;
 pub use crate::inline_assistant::InlineAssistant;
 
 actions!(