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

nanobot/channels/msteams.py

@@ -32,7 +32,10 @@ from nanobot.channels.base import BaseChannel
 from nanobot.config.paths import get_workspace_path
 from nanobot.config.schema import Base
 
-MSTEAMS_AVAILABLE = importlib.util.find_spec("jwt") is not None
+MSTEAMS_AVAILABLE = (
+    importlib.util.find_spec("jwt") is not None
+    and importlib.util.find_spec("cryptography") is not None
+)
 
 if TYPE_CHECKING:
     import jwt