revert: keep Google Chat OAuth secret + active_provider profile-scoped (#39398)

* Revert "fix(gateway): anchor Google Chat OAuth client secret to default Hermes root"

This reverts commit fff0561441.

* Revert "fix(cli): honor global-root active_provider fallback for named profiles"

This reverts commit 3858cf4307.

* docs(google_chat): describe OAuth client secret as profile-scoped, not host-wide

The setup docs, oauth docstring, and the adapter's 'no credentials'
error message all described the Google Chat OAuth client secret as
host-wide shared infrastructure. That contradicts profile isolation:
profiles are separate auth boundaries, so two profiles can point at
different Google OAuth apps / accounts. Reword all three to say the
secret is profile-scoped and each profile registers its own.

plugins/platforms/google_chat/adapter.py

@@ -1390,7 +1390,7 @@ class GoogleChatAdapter(BasePlatformAdapter):
         if arg == "start":
             if not oauth_helper._client_secret_path().exists():
                 await _reply(
-                    "⚠️ No client credentials stored on the host. Send "
+                    "⚠️ No client credentials stored for this profile. Send "
                     "`/setup-files` (no args) for setup instructions."
                 )
                 return True