refactor(channels): deslop tier-2 channel plugins third pass (#161817)

refactor(channels): deslop tier-2 channel plugins third pass (#161817)

Consolidate existing SDK and local owners across nine channel plugins while preserving transport, authorization, configuration, persistence, and capability contracts. Remove 241 net production lines and correct Teams QA direct/group conversation metadata, with a loopback transport regression.

Validation: 694 focused tests, 1,202 plugin contract and import-boundary tests, both cycle checks at zero, and independent review clean through P2. The starting base's stale Gateway max-lines entry is already fixed on main in #161802.