[data grid] imeFastEdit: text inserted without a composition is dropped #24063

Steps to reproduce

Uses experimentalFeatures.imeFastEdit from #23910.

1. Render a DataGrid with an editable string column and experimentalFeatures={{ imeFastEdit: true }}.
2. Click a cell.
3. Insert text without a composition:
   • The emoji picker (Ctrl+Cmd+Space on macOS, Win+. on Windows) or dictation.
   • An IME that commits a character directly, for example ， in Chinese Pinyin on macOS. The keydown has keyCode 229 and key: '，', and no compositionstart follows.

Current behavior

The input event reaches the hidden editor, but edit mode does not start and the text is lost.

The last row only breaks with the flag.

Expected behavior

With the flag, the hidden editor already receives the text. An input event in view mode starts edit mode with the inserted text, as a printable key does.

Root cause

• useGridCellEditing and useGridRowEditing ignore an IME keydown (isImeKeyDown) from the hidden editor. They expect compositionstart to start edit mode.
• handleChange in GridEditInputCell returns early for an event from the hidden editor while the cell is in view mode.

Without a composition, nothing starts edit mode.

Context

Found while reviewing #23910. Reproduced in Chromium with CDP (Input.dispatchKeyEvent and Input.insertText) on the pkg.pr.new build at bed2bcea4. Not yet checked with a real Pinyin IME.

Search keywords: IME, imeFastEdit, emoji picker, dictation, Pinyin, keyCode 229, edit mode