terminal: Implement basic Japanese IME support on macOS (#29879)

## Description

This PR implements basic support for Japanese Input Method Editors
(IMEs) in the Zed terminal on macOS, addressing issue #9900. Previously,
users had to switch input modes to confirm Japanese text, and pre-edit
(marked) text was not displayed.

With these changes:

- **Marked Text Display:** Pre-edit text (e.g., underlined characters
during Japanese composition) is now rendered directly in the terminal at
the cursor's current position.
- **Composition Confirmation:** Pressing Enter correctly finalizes the
IME composition, clears the marked text, and sends the confirmed string
to the underlying PTY process. This allows for a more natural input flow
similar to other macOS applications like iTerm2.
- **State Management:** IME state (marked text and its selected range
within the marked text) is now managed within the `TerminalView` struct.
- **Input Handling:** `TerminalInputHandler` has been updated to
correctly process IME callbacks (`replace_and_mark_text_in_range`,
`replace_text_in_range`, `unmark_text`, `marked_text_range`) by
interacting with `TerminalView`.
- **Painting Logic:** `TerminalElement::paint` now fetches the marked
text and its range from `TerminalView` and renders it with an underline.
The standard terminal cursor is hidden when marked text is present to
avoid visual clutter.
- **Candidate Window Positioning:**
`TerminalInputHandler::bounds_for_range` now attempts to provide more
accurate bounds for the IME candidate window by using the actual painted
bounds of the pre-edit text, falling back to a cursor-based
approximation if necessary.

This significantly improves the usability of the Zed terminal for users
who need to input Japanese characters, bringing the experience closer to
system-standard IME behavior.

## Movies


https://github.com/user-attachments/assets/be6c7597-7b65-49a6-b376-e1adff6da974

---

Closes #9900

Release Notes:

- **Terminal:** Implemented basic support for Japanese Input Method
Editors (IMEs) on macOS. Users can now see pre-edit (marked) text as
they type Japanese and confirm their input with the Enter key directly
in the terminal. This provides a more natural and efficient experience
for Japanese language input. (Fixes #9900)

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/terminal_view/src/terminal_view.rs

@@ -52,7 +52,7 @@ use zed_actions::assistant::InlineAssist;
 
 use std::{
     cmp,
-    ops::RangeInclusive,
+    ops::{Range, RangeInclusive},
     path::{Path, PathBuf},
     rc::Rc,
     sync::Arc,
@@ -126,6 +126,8 @@ pub struct TerminalView {
     scroll_handle: TerminalScrollHandle,
     show_scrollbar: bool,
     hide_scrollbar_task: Option<Task<()>>,
+    marked_text: Option<String>,
+    marked_range_utf16: Option<Range<usize>>,
     _subscriptions: Vec<Subscription>,
     _terminal_subscriptions: Vec<Subscription>,
 }
@@ -218,6 +220,8 @@ impl TerminalView {
             show_scrollbar: !Self::should_autohide_scrollbar(cx),
             hide_scrollbar_task: None,
             cwd_serialized: false,
+            marked_text: None,
+            marked_range_utf16: None,
             _subscriptions: vec![
                 focus_in,
                 focus_out,
@@ -227,6 +231,45 @@ impl TerminalView {
         }
     }
 
+    /// Sets the marked (pre-edit) text from the IME.
+    pub(crate) fn set_marked_text(
+        &mut self,
+        text: String,
+        range: Range<usize>,
+        cx: &mut Context<Self>,
+    ) {
+        self.marked_text = Some(text);
+        self.marked_range_utf16 = Some(range);
+        cx.notify();
+    }
+
+    /// Gets the current marked range (UTF-16).
+    pub(crate) fn marked_text_range(&self) -> Option<Range<usize>> {
+        self.marked_range_utf16.clone()
+    }
+
+    /// Clears the marked (pre-edit) text state.
+    pub(crate) fn clear_marked_text(&mut self, cx: &mut Context<Self>) {
+        if self.marked_text.is_some() {
+            self.marked_text = None;
+            self.marked_range_utf16 = None;
+            cx.notify();
+        }
+    }
+
+    /// Commits (sends) the given text to the PTY. Called by InputHandler::replace_text_in_range.
+    pub(crate) fn commit_text(&mut self, text: &str, cx: &mut Context<Self>) {
+        if !text.is_empty() {
+            self.terminal.update(cx, |term, _| {
+                term.input(text.to_string());
+            });
+        }
+    }
+
+    pub(crate) fn terminal_bounds(&self, cx: &App) -> TerminalBounds {
+        self.terminal.read(cx).last_content().terminal_bounds
+    }
+
     pub fn entity(&self) -> &Entity<Terminal> {
         &self.terminal
     }