Add a typed REST request/response contract for mscolab (mslib/mscolab/api/)

[ReimarBauer]

REST/socket payloads between mslib/mscolab (server) and mslib/msui/mscolab.py (client) are ad-hoc dicts with no machine-checkable link between a client call site and its server route. A rename or shape change on one side currently has no compiler/linter/test signal on the other

• This task is a wire-format migration only, behavior must not change.
• Create mslib/mscolab/api/ package: schemas.py (stdlib @dataclass request/response shapes per REST route — no new runtime dependency), endpoints.py (named endpoint registry, replacing bare strings like "create_operation").
• Migrate blueprint handlers and their client call sites route-by-route to build from / parse into these dataclasses, for every route that has a real msui caller. Convert any duplicate ad-hoc call sites onto the same schema as they're found.
• Add tests/_test_api/ with a round-trip test per schema (serialize → parse → compare), wired into test-fast so it stays a fast tripwire.
• Rework the server-isolation import-linter contract in setup.cfg so mslib.msui can reach mslib.mscolab.api without a growing list of per-symbol exceptions.
• Move mslib/mscolab/events.py and message_type.py into mslib/mscolab/api/ alongside the new modules — one clearly-named location for "this is the contract" instead of scattered files.
• Mark pre-existing bugs when you find them through the refactoring. by "NOTE: pre-existing bug"