Recipient chips could already be dragged between the To/Cc/Bcc fields, but a
drop always appended and same-field drops were a no-op, so recipients could not
be rearranged without deleting and re-adding them.
Add positional drag-and-drop: while dragging a chip, an insertion caret shows
the gap it would land in (based on which half of the hovered chip the pointer
is over, mirrored for RTL); dropping inserts it there.
- same-field drop reorders the chip locally (via onChipsChange), using the
source index carried in the drag payload (fromIndex) and adjusting for the
removal shift; dropping onto its own position is a no-op;
- cross-field drop inserts at the drop position: handleMoveChip gained an
optional toIndex (omitted = append, e.g. dropping onto a hidden Cc/Bcc
button, preserving existing behaviour);
- per-chip onDragOver computes the target gap; the container handles the
trailing gap (past the last chip / over the input).
No new user-facing strings (the caret is purely visual), so no locale changes.
Tests (components/email/__tests__/recipient-chip-drag.test.tsx): reorder to end
/ front, self-drop no-op, cross-field positional insert, and caret visibility.
Also add the missing findComposeIdentityId export to the reply-identity mock in
the recipient drag/paste suites so <EmailComposer> mounts in compose mode.
