Guard drops whose position resolves outside of the editor

[janpaepke]

Description
Dropping into <Editable> throws Cannot resolve a Slate point from DOM point when the browser resolves the drop position to a node outside of the editor, and the drop is lost. onDrop checks that event.target is inside the editor, then resolves the range from clientX/clientY via caretRangeFromPoint. The two can disagree: WebKit dispatches drop to the element captured at the last dragover while the coordinates are the drop's own, so a page scroll during the drag lands the caret on neighbouring text. Seen in production in Safari 18.6 on macOS, where the caret resolved to the end of a <label> above the editor.

With this change the dropped data is inserted at the current selection (or the end of the document when there is none), and an internal drag without a resolvable target leaves its content where it is.

Issue
No dedicated issue. Related: #5107 (the same caretRangeFromPoint fragility inside shadow DOM).

Example

// before: throws when the caret position lies outside of the editor
const range = ReactEditor.findEventRange(editor, event)

// after: opt into `null` instead, like `toSlatePoint` / `toSlateRange`
const range = ReactEditor.findEventRange(editor, event, { suppressThrow: true })
if (range) Transforms.select(editor, range)

Before: the handler threw, nothing was inserted and the error surfaced as uncaught. After: the data lands at the selection.

Context

• DOMEditor.findEventRange gets an optional suppressThrow, following toSlatePoint/toSlateRange and the option useSelected gained in fix(slate-react): prevent useSelected from throwing when element is removed #6073. It forwards the flag to toSlateRange and returns null for the two early throws as well.
• Unlike toSlatePoint/toSlateRange, the options object is optional and the generic defaults to false, so existing callers keep the plain Range return type; only callers passing true see Range | null.
• <Editable>'s onDrop branches on the result: with a range, behaviour is unchanged; without one and an internal drag, it returns after preventDefault; otherwise it selects the end of the document when nothing is selected and calls insertData as before. The children.length > 0 guard mirrors focus.
• Tests: four onDrop cases dispatching a real drop event through React with a stubbed caretRangeFromPoint (jsdom implements neither it nor DataTransfer).

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test:jest (101 passed).
• The linter passes (yarn lint:typescript; eslint and prettier on the changed files).
• The relevant examples still work. (not run; the resolvable-position path is unchanged)
• Changeset added (slate-dom minor, slate-react patch).

[changeset-bot]

🦋 Changeset detected

Latest commit: ed1bde2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
slate-dom	Minor
slate-react	Minor

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