Same class of bug as the LINE adapter (NS-603): defaulting the webhook
bind to "0.0.0.0" (or hardcoding it) binds IPv4 ONLY, so the listener
is unreachable over IPv6-only private networks such as Fly.io 6PN.
- wecom callback_adapter: DEFAULT_HOST None; config.py env seed no
longer forces 0.0.0.0 when WECOM_CALLBACK_HOST is unset.
- msgraph_webhook: DEFAULT_HOST None; the allowed_source_cidrs
requirement still fires for the all-interfaces default (host=None is
treated as network-accessible).
- whatsapp_cloud: DEFAULT_WEBHOOK_HOST None.
- teams: hardcoded 0.0.0.0 TCPSite bind → _DEFAULT_HOST=None with new
TEAMS_HOST / extra.host override (mirrors LINE_HOST pattern).
- telegram: hardcoded listen="0.0.0.0" → default "" (tornado
bind_sockets opens one socket per address family; verified against
PTB 22.6/tornado) with new TELEGRAM_WEBHOOK_HOST / extra.webhook_host
override.
Explicit host overrides everywhere are preserved; empty/unset collapses
to the dual-stack default. "::" remains a bad substitute on
bindv6only=1 hosts (see LINE adapter comment).
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