Fix stale CodeEditor findController listener swap

[SebTardif]

Summary

• remove _updateWidget from the previous findController before swapping controllers in CodeEditor.didUpdateWidget
• add regression tests for stale notifications after a controller swap and after the editor is removed

Problem

When CodeEditor swaps to a new findController, didUpdateWidget() overwrites _findController before removing _updateWidget. That means the remove call targets the new controller instead of the old one, leaving the old controller able to keep rebuilding the editor after it has been detached.

Reproduction

1. Build a CodeEditor with oldFindController
2. Rebuild it with newFindController
3. Trigger oldFindController.findMode()

Before this change, the stale controller still notifies the editor and causes an extra rebuild. If the editor is removed from the tree first, the stale controller can also trigger setState() after dispose.

Fix

Store the previous controller in a local variable, detach _updateWidget from that instance, then assign and subscribe the new controller.

Verification

• red step: reverted the production fix, each regression test failed
• green step: restored the fix, both regression tests passed
• flutter test test/code_search_controller_test.dart
• flutter analyze --no-fatal-infos lib/src/code_editor.dart test/code_search_controller_test.dart

Bug origin

• introduced in b35627f on 2024-02-05

Follow-up improvements in this PR

• closes Add regression coverage for stale findController callbacks after editor disposal #108