v4.0.0a1: Fourst Contact

Document v3->v4 removals and add upgrade-reality tests (#4585)

* Document v3->v4 removals and add upgrade-reality tests

* Check canonical imports in a clean subprocess to avoid suite import pollution

* Address review: import_server semantics note, pin traversal error, drop redundant import

* Address review round 2: real screening test, Depends factory, remove_tool/create_proxy notes

* Validate canonical imports in-process; fix lifespan/timeout/error-code/starlette doc notes

* Reconcile with fastmcp.types trim: import protocol types from mcp_types

* Record v4 release codename arc in dev notes