multi_buffer: Fix up some anchor checks (#43454)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/assistant_text_thread/src/assistant_text_thread_tests.rs

@@ -880,10 +880,9 @@ async fn test_random_context_collaboration(cx: &mut TestAppContext, mut rng: Std
                     let num_sections = rng.random_range(0..=3);
                     let mut section_start = 0;
                     for _ in 0..num_sections {
-                        let mut section_end = rng.random_range(section_start..=output_text.len());
-                        while !output_text.is_char_boundary(section_end) {
-                            section_end += 1;
-                        }
+                        let section_end = output_text.floor_char_boundary(
+                            rng.random_range(section_start..=output_text.len()),
+                        );
                         events.push(Ok(SlashCommandEvent::StartSection {
                             icon: IconName::Ai,
                             label: "section".into(),

crates/assistant_text_thread/src/text_thread.rs

@@ -16,6 +16,7 @@ use gpui::{
     App, AppContext as _, Context, Entity, EventEmitter, RenderImage, SharedString, Subscription,
     Task,
 };
+use itertools::Itertools as _;
 use language::{AnchorRangeExt, Bias, Buffer, LanguageRegistry, OffsetRangeExt, Point, ToOffset};
 use language_model::{
     LanguageModel, LanguageModelCacheConfiguration, LanguageModelCompletionEvent,
@@ -1853,14 +1854,17 @@ impl TextThread {
                         }
 
                         if ensure_trailing_newline
-                            && buffer.contains_str_at(command_range_end, "\n")
+                            && buffer
+                                .chars_at(command_range_end)
+                                .next()
+                                .is_some_and(|c| c == '\n')
                         {
-                            let newline_offset = insert_position.saturating_sub(1);
-                            if buffer.contains_str_at(newline_offset, "\n")
+                            if let Some((prev_char, '\n')) =
+                                buffer.reversed_chars_at(insert_position).next_tuple()
                                 && last_section_range.is_none_or(|last_section_range| {
                                     !last_section_range
                                         .to_offset(buffer)
-                                        .contains(&newline_offset)
+                                        .contains(&(insert_position - prev_char.len_utf8()))
                                 })
                             {
                                 deletions.push((command_range_end..command_range_end + 1, ""));