The LINE adapter's webhook server defaulted to host="0.0.0.0", which
binds IPv4 ONLY. On IPv6-only private networks — notably Fly.io 6PN,
where the hosted edge router reverse-proxies LINE ingest to
<app>.internal:8646 over an fdaa: IPv6 address — nothing is listening
on the dialed address: connection refused → customer-visible 502 when
LINE's console verifies the webhook (NS-603).
This is the same bug the generic webhook adapter fixed in d542894ad;
the LINE adapter was never updated to match. Fix mirrors that commit:
- DEFAULT_HOST = None → asyncio/aiohttp create_server binds one socket
per address family (v4 + v6), regardless of the bindv6only sysctl.
"::" is NOT a valid substitute — Fly machines set bindv6only=1, so
it would yield an IPv6-only socket and break IPv4 loopback probes.
- Empty-string host collapses to None; LINE_HOST/extra.host still pin
a specific bind address.
- reuse_address=False scoped to macOS only (BSD wildcard-socket
traffic-splitting footgun), mirroring 9420ad946.
- The three outbound-media guards compared webhook_host == "0.0.0.0"
by string equality; extracted to _missing_public_url() which treats
None/0.0.0.0/::/"" as "no fetchable hostname" so the LINE_PUBLIC_URL
requirement still fires under the new default. _media_url() falls
back to 127.0.0.1 instead of interpolating 'None' into URLs.
Tests: dual-stack default decision table (default None, empty→None,
pinned preserved, LINE_HOST override), behavioural both-families bind
proof via runner.addresses, and the media public-URL guard matrix.
88 passed, ruff clean.
Companion router fix (hermes-agent-router) routes /webhooks/line and
/line/webhook|/line/media to :8646 over 6PN; both are needed for
end-to-end hosted LINE delivery.
Fixes NS-603
e24bb0b42f