fix: guard delayed drag autoscroll after dispose

[SebTardif]

Summary

Delayed drag autoscroll callbacks can run after the CodeEditor is disposed. They call render / ensureRender, which cast findRenderObject() with as _CodeFieldRender. When currentContext is null after dispose, that cast throws and crashes the app.

This change:

• Returns early when !mounted in the gesture detector autoscroll loop
• Uses a safe is! _CodeFieldRender check instead of a force cast before calling autoScrollWhenDragging (desktop drag + mobile handle drag paths)

Failure scenario

1. User starts a text selection drag (or drags a mobile selection handle).
2. Autoscroll schedules Future.delayed(100ms, ...).
3. The editor is removed from the tree before the callback runs.
4. The callback dereferences a null render object via as _CodeFieldRender and crashes.

Bug origin

_autoScrollWhenDragging has used Future.delayed since the selection gesture code was introduced (MegatronKing, 2024-02). The mobile handle paths use the same pattern with ensureRender.

Related

• Audit finding F004 (dispose-time cast crash on delayed autoscroll)
• Prior merged fix for a related dispose/listener issue: Fix stale CodeEditor findController listener swap #107

Test plan

• Diff limited to the three delayed autoscroll callbacks
• Manual: start drag selection, dispose the editor mid-drag, confirm no crash
• CI analysis on the package

No automated widget test added: this path depends on Flutter render objects and delayed callbacks; the fix is the same null-safe pattern already used elsewhere in _MobileSelectionOverlayController for optional render lookups.