agent: Make assistant panel input size configurable (#37975)

Release Notes:

- Added the `agent. message_editor_min_lines `setting to allow users to
customize the agent panel message editor default size by using a
different minimum number of lines.

<img width="800" height="1316" alt="Screenshot 2025-09-11 at 5 47 18 pm"
src="https://github.com/user-attachments/assets/20990b90-c4f9-4f5c-af59-76358642a273"
/>

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

docs/src/ai/agent-settings.md

@@ -170,6 +170,21 @@ The default value is `false`.
 
 > This setting is available via the Agent Panel's settings UI.
 
+### Message Editor Size
+
+Use the `message_editor_min_lines` setting to control minimum number of lines of height the agent message editor should have.
+It is set to `4` by default, and the max number of lines is always double of the minimum.
+
+```json
+{
+  "agent": {
+    "message_editor_min_lines": 4
+  }
+}
+```
+
+> This setting is currently available only in Preview.
+
 ### Modifier to Send
 
 Make a modifier (`cmd` on macOS, `ctrl` on Linux) required to send messages.