agent_ui: Use agent font size tokens for thread markdown rendering (#41610)

Release Notes:

- N/A

--- 

Previously, agent markdown rendering used hardcoded font sizes
(TextSize::Default and TextSize::Small) which ignored the
agent_ui_font_size and agent_buffer_font_size settings. This updates the
markdown style to respect these settings.

This pull request adds support for customizing the font size of code
blocks in agent responses, making it possible to set a distinct font
size for code within the agent panel. The changes ensure that if the new
setting is not specified, the font size will fall back to the agent UI
font size, maintaining consistent appearance.

(I am a frontend developer without any Rust knowledge so this is
co-authored with Claude Code)


**Theme settings extension:**

* Added a new `agent_buffer_code_font_size` setting to
`ThemeSettingsContent`, `ThemeSettings`, and the default settings JSON,
allowing users to specify the font size for code blocks in agent
responses.
[[1]](diffhunk://#diff-a3bba02a485aba48e8e9a9d85485332378aa4fe29a0c50d11ae801ecfa0a56a4R69-R72)
[[2]](diffhunk://#diff-aed3a9217587d27844c57ac8aff4a749f1fb1fc5d54926ef5065bf85f8fd633aR118-R119)
[[3]](diffhunk://#diff-42e01d7aacb60673842554e30970b4ddbbaee7a2ec2c6f2be1c0b08b0dd89631R82-R83)
* Updated the VSCode import logic to recognize and import the new
`agent_buffer_code_font_size` setting.

**Font size application in agent UI:**

* Modified the agent UI rendering logic in `thread_view.rs` to use the
new `agent_buffer_code_font_size` for code blocks, and to fall back to
the agent UI font size if unset.
[[1]](diffhunk://#diff-f73942e8d4f8c4d4d173d57d7c58bb653c4bb6ae7079533ee501750cdca27d98L5584-R5584)
[[2]](diffhunk://#diff-f73942e8d4f8c4d4d173d57d7c58bb653c4bb6ae7079533ee501750cdca27d98L5596-R5598)
* Implemented a helper method in `ThemeSettings` to retrieve the code
block font size, with fallback logic to ensure a value is always used.
* Updated the settings application logic to propagate the new code block
font size setting throughout the theme system.


### Example Screenshots
![Screenshot 2025-10-31 at 12 38
28](https://github.com/user-attachments/assets/cbc34232-ab1f-40bf-a006-689678380e47)
![Screenshot 2025-10-31 at 12 37
45](https://github.com/user-attachments/assets/372b5cf8-2df8-425a-b052-12136de7c6bd)

---------

Co-authored-by: Danilo Leal <[email protected]>

Author: machr

crates/agent_ui/src/acp/thread_view.rs

@@ -5596,7 +5596,7 @@ fn default_markdown_style(
     let theme_settings = ThemeSettings::get_global(cx);
     let colors = cx.theme().colors();
 
-    let buffer_font_size = TextSize::Small.rems(cx);
+    let buffer_font_size = theme_settings.agent_buffer_font_size(cx);
 
     let mut text_style = window.text_style();
     let line_height = buffer_font_size * 1.75;
@@ -5608,9 +5608,9 @@ fn default_markdown_style(
     };
 
     let font_size = if buffer_font {
-        TextSize::Small.rems(cx)
+        theme_settings.agent_buffer_font_size(cx)
     } else {
-        TextSize::Default.rems(cx)
+        theme_settings.agent_ui_font_size(cx)
     };
 
     let text_color = if muted_text {