* Make the SDK seam the root of FastMCP middleware dispatch (D3)
Notifications, cancellations, and malformed/unroutable messages now reach
on_message/on_request/on_notification at the SDK seam. Component methods keep
their interior dispatch (typed hooks, tool-exception visibility) unchanged; the
seam covers only messages the interior never dispatches, so each hook fires once.
* Document the middleware seam coverage and suspend semantics (D3)
* Align seam docs and ask-visibility test with the result-cycle MRTR model
An InputRequiredResult is the full result of a complete request->response
cycle, not a suspension: component hooks observe an asking round's
InputRequiredToolResult as an ordinary return value.
* Replace 'seam' language with plain dispatch terminology
* Keep the raw middleware __call__ signature; forward middleware message edits
* Cover fires-once across an MRTR continuation round
* Align cherry-picked coverage test with renamed recorder
* Rewrite only the message, never the dispatch destination
