fix(contracts): params validation rejects only unknown keys; accepted params + results are checked after the handler

Handlers own their documented domain codes (4006 missing session_id, 4015 bad
url, 4009 orphan claim); the contract's job on the way in is the one check no
handler performs — an unknown key (4000 with the key path). Missing/mistyped
fields are re-checked AFTER a successful handler answer under the strict
test policy, so a contract narrower than the wire still fails the suite.
Two models widened from the suite: SeedMessage (clients forward stored rows
verbatim), tool.complete.args (mirrored child rows omit it). Tests that
drove session.activate with prompt params (and vice versa) or stubbed
_live_session_payload with a bare {session_id} now send the real shapes.

tui_gateway/server_requests.py

@@ -92,7 +92,7 @@ def _register(req: ServerRequest) -> None:
         raise RuntimeError(f"server request {req.method!r} has no contract in tui_gateway/contracts")
     _, problem = contracts.validate_params(contract, {"session_id": req.sid, **req.params})
     if problem is not None:
-        raise ValueError(problem)
+        raise ValueError(problem)  # a key the renderer's typed handler would never read: our bug
     with _lock:
         _open[req.id] = req
     _write(req.frame())