fix(msteams): harden availability check and migrate docs to README

- Check both jwt and cryptography in MSTEAMS_AVAILABLE guard so
  partial installs fail early with a clear message instead of at runtime
- Add aclose() to test FakeHttpClient so stop() won't crash
- Move MSTEAMS.md into README.md following the same details/summary
  pattern used by every other channel
- Note in README that validateInboundAuth defaults to false

tests/test_msteams.py

@@ -52,6 +52,9 @@ class FakeHttpClient:
         self.calls.append((url, kwargs))
         return FakeResponse(self.payload, should_raise=self.should_raise)
 
+    async def aclose(self):
+        pass
+
 
 @pytest.fixture
 def make_channel(tmp_path, monkeypatch):