After context compression, the parent session holds pre-compression messages
and a child (or deeper descendant) holds the continuation.
resolve_resume_session_id() short-circuited when the input session already
had messages (row is not None -> return session_id), causing REST API
endpoints, gateway resume, and CLI resume to serve stale parent messages.
Remove the early-return. Walk the full descendant chain, record the
deepest node that has messages (best), and return best if not None
else the original session_id (preserving the empty-chain fallback).
Callers (api_server.py, web_server.py, cli_agent_setup_mixin.py,
cli_commands_mixin.py) all use the resolved != input -> redirect pattern
and are transparent to this change.
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