test(google-chat): cover relay-declared sender_type honoring

Adds five regression tests for the Format 3 (Cloud Run relay) envelope
path:

- test_relay_flat_honors_declared_sender_type_bot: BOT sender_type
  propagates to msg['sender']['type'].
- test_relay_flat_defaults_sender_type_human_when_absent: backward
  compat \u2014 missing field still flows as HUMAN.
- test_relay_flat_coerces_unknown_sender_type_to_human: defensive
  coercion \u2014 strip+upper normalizes whitespace/case, anything outside
  {HUMAN, BOT} falls back to HUMAN.
- test_relay_flat_bot_sender_is_filtered_end_to_end: end-to-end
  through _on_pubsub_message \u2014 a relay envelope with sender_type=BOT
  is dropped by the BOT self-filter without dispatch.
- test_relay_flat_human_sender_dispatches: end-to-end negative
  control \u2014 human relay envelopes still reach the agent loop.

Also clarifies the operator contract in the adapter comment: the
relay must forward upstream sender.type as envelope.sender_type,
otherwise bot replies forwarded as HUMAN cannot be distinguished
from genuine humans by this filter.

plugins/platforms/google_chat/adapter.py

@@ -1018,6 +1018,11 @@ class GoogleChatAdapter(BasePlatformAdapter):
             # impersonate any allowlisted user without ever being marked
             # as a bot. Default to "HUMAN" for backward compatibility when
             # the relay does not provide the field.
+            #
+            # Operator contract: the relay MUST forward sender.type from
+            # the upstream Chat event as ``sender_type``. Relays that
+            # forward bot replies as HUMAN (or omit the field) cannot be
+            # distinguished from genuine humans here.
             sender_type_raw = (envelope.get("sender_type") or "HUMAN")
             sender_type = str(sender_type_raw).strip().upper() or "HUMAN"
             if sender_type not in {"HUMAN", "BOT"}: