Make transcript mouse scroll speed configurable (#50209)

## What changed

Add `tui.mouse_scroll_speed` as a finite positive multiplier for transcript mouse wheel scrolling, including transcript overlays and session previews. Change the default from three rows to one row per wheel event; set the multiplier to `3.0` to retain the previous speed.

Fractional values accumulate movement across events and discard the remainder when direction reverses. For example, `0.5` scrolls one row every two events. Following the latest output pauses only when a whole row moves.

## Testing

Add tests for configuration validation, integer and fractional scroll speeds, direction changes, decimal accumulation, and following behavior. Extend local settings coverage to include a command-line override for the new setting.

GitOrigin-RevId: d355e246fd943415f0ce5a6affe849f77f3615e6

codex-rs/thread-manager-sample/src/main.rs

@@ -290,6 +290,7 @@ async fn new_config(
         tui_auto_recap: true,
         model_availability_nux: ModelAvailabilityNuxConfig::default(),
         tui_fullscreen_transcript: false,
+        tui_mouse_scroll_speed: None,
         tui_copy_on_select: Default::default(),
         tui_right_click_paste: Default::default(),
         tui_alternate_screen: AltScreenMode::Auto,