Reuse prompt editor across buffer and terminal assist (#22188)

Builds on #22160 and extracts
the rest of `PromptEditor` so it can be shared across terminal and
inline assistants. This will help avoid the UI drifting as we have
already observed.

Note: This is mostly a mechanical refactor. I imagine some things could
be factored in a better way by someone with more context, but I think
this is a good start.

Release Notes:

- N/A

---------

Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/assistant2/src/assistant.rs

@@ -1,6 +1,7 @@
 mod active_thread;
 mod assistant_panel;
 mod assistant_settings;
+mod buffer_codegen;
 mod context;
 mod context_picker;
 mod context_store;
@@ -10,6 +11,7 @@ mod inline_prompt_editor;
 mod message_editor;
 mod prompts;
 mod streaming_diff;
+mod terminal_codegen;
 mod terminal_inline_assistant;
 mod thread;
 mod thread_history;