Add dm_policy and group_policy to the WhatsApp adapter, bringing parity
with WeCom/Weixin/QQ. Allows independent control of DM and group access:
disable DMs entirely, allowlist specific senders/groups, or keep open.
- dm_policy: open (default) | allowlist | disabled
- group_policy: open (default) | allowlist | disabled
- Config bridging for YAML → env vars
- 22 tests covering all policy combinations
Backward compatible — defaults preserve existing behavior.
Cherry-picked from PR #11597 by @MassiveMassimo.
Dropped the run.py group auth bypass (would have skipped user auth
for ALL platforms, not just WhatsApp).