Tidy up diagnostics more (#29629)

- Stop merging same row diagnostics
- (for Rust) show code fragments surrounded by `'s in monospace

Co-authored-by: Serge Radinovich <sergeradinovich@gmail.com>

Closes #29362

Release Notes:

- diagnostics: Diagnostics are no longer merged when they're on the same
line
- rust: Diagnostics now show code snippets in monospace font:

<img width="551" alt="Screenshot 2025-04-29 at 16 13 45"
src="https://github.com/user-attachments/assets/d289be31-717d-404f-a76a-a0cda3e96fbe"
/>

Co-authored-by: Serge Radinovich <sergeradinovich@gmail.com>

crates/editor/src/hover_popover.rs

@@ -633,7 +633,7 @@ pub fn hover_markdown_style(window: &Window, cx: &App) -> MarkdownStyle {
         base_text_style,
         code_block: StyleRefinement::default().my(rems(1.)).font_buffer(cx),
         inline_code: TextStyleRefinement {
-            background_color: Some(cx.theme().colors().background),
+            background_color: Some(cx.theme().colors().editor_background.opacity(0.5)),
             font_family: Some(buffer_font_family),
             font_fallbacks: buffer_font_fallbacks,
             ..Default::default()