agent: Make the panel's textarea font size be controlled by buffer_font_size (#38726)

Closes https://github.com/zed-industries/zed/issues/37882

Previously, every piece of text in the agent panel was controlled by
`agent_font_size`. Although it is nice to only have one setting to tweak
that, it could be a bit misleading particularly because we use
monospaced and sans-serif fonts for different elements in the panel. Any
editor/textarea in the panel, whehter it is the main message editor or
the previous message editor, uses the buffer font. Therefore, I think it
is reasonable to expect that tweaking `buffer_font_size` would also
change the agent panel's usage of buffer fonts.

With this change, regular buffers and the agent panel's message editor
will always have the same size.

Release Notes:

- agent: Made the agent panel's textarea font size follow the font size
of regular buffers. They're now both controlled by the
`buffer_font_size` setting.

docs/src/ai/agent-settings.md

@@ -123,6 +123,20 @@ You can choose between `thread` (the default) and `text_thread`:
 }
 ```
 
+### Font Size
+
+Use the `agent_font_size` setting to change the font size of rendered agent responses in the panel.
+
+```json
+{
+  "agent": {
+    "agent_font_size": 18
+  }
+}
+```
+
+> (Preview Version Note): Editors in the Agent Panel—whether that is the main message textarea or previous messages—use monospace fonts and therefore, are controlled by the `buffer_font_size` setting, which is defined globally in your `settings.json`.
+
 ### Auto-run Commands
 
 Control whether you want to allow the agent to run commands without asking you for permission.