terminal: Match trait bounds with terminal input (#31441)

The core change here is the following:

```rust
fn write_to_pty(&self, input: impl Into<Vec<u8>>);

// into
fn write_to_pty(&self, input: impl Into<Cow<'static, [u8]>>);
```

This matches the trait bounds that's used by the Alacritty crate. We are
now allowed to effectively pass `&'static str` instead of always needing
a `String`.

The main benefit comes from making the `to_esc_str` function return a
`Cow<'static, str>` instead of `String`. We save an allocation in the
following instances:

- When the user presses any special key that isn't alphanumerical (in
the terminal)
- When the uses presses any key while a modifier is active (in the
terminal)
- When focusing/un-focusing the terminal
- When completing or undoing a terminal transaction
- When starting a terminal assist

This basically saves us an allocation on **every key** press in the
terminal.

NOTE: This same optimization can be done for **nearly all** keypresses
in the entirety of Zed by changing the signature of the `Keystroke`
struct in gpui. If the Zed team is interested in a PR for it, let me
know.

Release Notes:

- N/A

crates/agent/src/terminal_codegen.rs

@@ -179,18 +179,17 @@ impl TerminalTransaction {
         // Ensure that the assistant cannot accidentally execute commands that are streamed into the terminal
         let input = Self::sanitize_input(hunk);
         self.terminal
-            .update(cx, |terminal, _| terminal.input(input));
+            .update(cx, |terminal, _| terminal.input(input.into_bytes()));
     }
 
     pub fn undo(&self, cx: &mut App) {
         self.terminal
-            .update(cx, |terminal, _| terminal.input(CLEAR_INPUT.to_string()));
+            .update(cx, |terminal, _| terminal.input(CLEAR_INPUT.as_bytes()));
     }
 
     pub fn complete(&self, cx: &mut App) {
-        self.terminal.update(cx, |terminal, _| {
-            terminal.input(CARRIAGE_RETURN.to_string())
-        });
+        self.terminal
+            .update(cx, |terminal, _| terminal.input(CARRIAGE_RETURN.as_bytes()));
     }
 
     fn sanitize_input(mut input: String) -> String {