Remove deprecated 2.x PATCH format

The 2.x PATCH API (Heading / Content-Insertion-Position headers) was
deprecated in version 3.0 and carried a sunset warning pointing at
version 4.0. Now that version 4.0 is released, this removes the
implementation entirely.

All PATCH requests are now handled exclusively by the 3.x path, which
requires Target-Type, Operation, and Target headers. The dispatcher
logic, _vaultPatchV2, findHeadingBoundary, getSplicePosition, the
HeadingBoundary type, and the three 2.x-specific error codes
(40050–40052) have all been deleted.

Unit and integration tests covering 2.x behaviour have been removed.
The OpenAPI description no longer mentions the deprecation notice.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>