refactor: MessageEvent to gateway/platforms/event.py; ElicitationHandler takes a call_context thunk

Breaks the two import cycles that forced Protocol stand-ins in the F821 sweep, so the two
sites now name the real types.

gateway/platforms/event.py (new leaf): MessageType, ProcessingOutcome, MessageEvent moved
out of base.py verbatim. Their only dependency is gateway.session.SessionSource; base.py
imported helpers.py at module level, so helpers could not name MessageEvent. Now
TextBatchAggregator is typed by the real MessageEvent. 249 importers repointed
(`from gateway.platforms.base import` -> `.event`, preserving each import's layout);
gateway.platforms.__init__ re-exports from .event. The three revert-scheduled PLUGIN-COMPAT
pointers that named these symbols (gateway.slash_commands → MessageType, dingtalk → MessageType,
photon → ProcessingOutcome) and their COMPAT_MANIFEST rows now target gateway.platforms.event.
Docs updated: ADDING_A_PLATFORM.md, adding-platform-adapters.md (en + zh-Hans).

tools/mcp_tool_sampling.py: ElicitationHandler no longer holds a back-reference to its
MCPServerTask (mcp_tool imports sampling, so the task type cannot be named there). It only
ever read owner._pending_call_context, so it takes `call_context: Callable[[], Context | None]`
and MCPServerTask passes `lambda: self._pending_call_context`. The consent call is one
`functools.partial`, run directly or inside the captured Context.

ty on the 11 touched production files vs origin/main: 0 new diagnostics, 14 resolved.
(The one `source: SessionSource = None` diagnostic moves with the class; typing it Optional
exposes ~60 unguarded call sites — separate follow-up.)

Tests: tests/gateway + tests/plugins + tests/tools + touched files, 18,235 passed; the 31
failures reproduce identically on origin/main (macOS /private/tmp, systemd socket,
long-path fixtures, live-service tests).

tests/gateway/relay/test_relay_multiplatform.py

@@ -100,7 +100,7 @@ def test_self_provision_loops_per_platform(monkeypatch):
 async def test_adapter_stamps_per_frame_platform_from_inbound(monkeypatch):
     """An inbound from a concrete platform makes the reply egress tagged for it."""
     from gateway.config import Platform, PlatformConfig
-    from gateway.platforms.base import MessageEvent, MessageType
+    from gateway.platforms.event import MessageEvent, MessageType
     from gateway.relay.adapter import RelayAdapter
     from gateway.relay.descriptor import CONTRACT_VERSION, CapabilityDescriptor
     from gateway.session import SessionSource