agent: Rename a number of constructs from Assistant to Agent (#30196)

This PR renames a number of constructs in the `agent` crate from the
"Assistant" terminology to "Agent".

Not comprehensive, but it's a start.

Release Notes:

- N/A

crates/agent/src/inline_prompt_editor.rs

@@ -1,4 +1,4 @@
-use crate::assistant_model_selector::{AssistantModelSelector, ModelType};
+use crate::agent_model_selector::{AgentModelSelector, ModelType};
 use crate::buffer_codegen::BufferCodegen;
 use crate::context::ContextCreasesAddon;
 use crate::context_picker::{ContextPicker, ContextPickerCompletionProvider};
@@ -42,7 +42,7 @@ pub struct PromptEditor<T> {
     context_store: Entity<ContextStore>,
     context_strip: Entity<ContextStrip>,
     context_picker_menu_handle: PopoverMenuHandle<ContextPicker>,
-    model_selector: Entity<AssistantModelSelector>,
+    model_selector: Entity<AgentModelSelector>,
     edited_since_done: bool,
     prompt_history: VecDeque<String>,
     prompt_history_ix: Option<usize>,
@@ -290,12 +290,12 @@ impl<T: 'static> PromptEditor<T> {
             PromptEditorMode::Terminal { .. } => "Generate",
         };
 
-        let assistant_panel_keybinding =
+        let agent_panel_keybinding =
             ui::text_for_action(&zed_actions::assistant::ToggleFocus, window, cx)
                 .map(|keybinding| format!("{keybinding} to chat ― "))
                 .unwrap_or_default();
 
-        format!("{action}… ({assistant_panel_keybinding}↓↑ for history)")
+        format!("{action}… ({agent_panel_keybinding}↓↑ for history)")
     }
 
     pub fn prompt(&self, cx: &App) -> String {
@@ -927,7 +927,7 @@ impl PromptEditor<BufferCodegen> {
             context_strip,
             context_picker_menu_handle,
             model_selector: cx.new(|cx| {
-                AssistantModelSelector::new(
+                AgentModelSelector::new(
                     fs,
                     model_selector_menu_handle,
                     prompt_editor.focus_handle(cx),
@@ -1098,7 +1098,7 @@ impl PromptEditor<TerminalCodegen> {
             context_strip,
             context_picker_menu_handle,
             model_selector: cx.new(|cx| {
-                AssistantModelSelector::new(
+                AgentModelSelector::new(
                     fs,
                     model_selector_menu_handle.clone(),
                     prompt_editor.focus_handle(cx),