Rely on model to determine indentation level and always rewrite the full line (#16145)

This PR simplifies our approach to indentation in the inline assistant
in hopes of improving our experience for Python. We tell the model to
generate the correct indentation in the prompt, and always start
generating at the start of the line. This may fall down for less capable
models, but I want to get a solid experience on the best models and then
figure the rest out later.

Also: We now prefer `./assets/prompts` as an overrides directory when
stdout is a PTY, so you can do `cargo run` and then iterate prompts for
the current run inside the current working copy.

cc @trishume @dsp-ant 

Release Notes:

- Zed now allows the model to control indentation when performing inline
transformation. We're hoping this improves the indentation experience in
Python and other indentation-sensitive languages, but it does require
more from the model.

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/zed/src/main.rs

@@ -187,7 +187,12 @@ fn init_common(app_state: Arc<AppState>, cx: &mut AppContext) -> Arc<PromptBuild
     );
     snippet_provider::init(cx);
     inline_completion_registry::init(app_state.client.telemetry().clone(), cx);
-    let prompt_builder = assistant::init(app_state.fs.clone(), app_state.client.clone(), cx);
+    let prompt_builder = assistant::init(
+        app_state.fs.clone(),
+        app_state.client.clone(),
+        stdout_is_a_pty(),
+        cx,
+    );
     repl::init(
         app_state.fs.clone(),
         app_state.client.telemetry().clone(),

crates/zed/src/zed.rs

@@ -1018,6 +1018,8 @@ fn open_settings_file(
 
 #[cfg(test)]
 mod tests {
+    use crate::stdout_is_a_pty;
+
     use super::*;
     use anyhow::anyhow;
     use assets::Assets;
@@ -3485,8 +3487,12 @@ mod tests {
                 app_state.fs.clone(),
                 cx,
             );
-            let prompt_builder =
-                assistant::init(app_state.fs.clone(), app_state.client.clone(), cx);
+            let prompt_builder = assistant::init(
+                app_state.fs.clone(),
+                app_state.client.clone(),
+                stdout_is_a_pty(),
+                cx,
+            );
             repl::init(
                 app_state.fs.clone(),
                 app_state.client.telemetry().clone(),