Stop Return in the search box from deleting the matched text

[HaozheZhang6]

Pressing Return in the Ctrl+F search box deletes the match instead of moving to the next one.

Repro on master:

1. put key=1 in the editor
2. Ctrl+F, type key
3. press Return

The document is now \n=1. Each further Return eats another match.

Cause: QLineEdit ignores Return after emitting returnPressed, and SearchWidget is a child of the editor, so the key propagates up to CodeEditor.keyPressEvent and QPlainTextEdit inserts a newline. highlight_current_match() puts the current match into the editor's text cursor via setTextCursor(), so that newline lands on a selection and replaces it.

Fix: accept Return/Enter in SearchWidget.keyPressEvent so it stops there. find_next() still runs from returnPressed, and Escape/F3 still reach the editor by propagation as before.

Test: test_search_return_does_not_edit_document sends real key events through the search input. It fails on master:

    import cadquery as cq
+   result =
-   result = cq.Workplane().box(10, 10, 10)
+   .Workplane().box(10, 10, 10)