Port of #59661 onto post-#102117 main. The refactor moved get_messages
into hermes_state_messages.py (SessionMessagesMixin) and made the REST
endpoint resolve the resume tip first, so the port adds the
include_ancestors kwarg to the new home — reusing _resume_lineage_ids
(same branch-session exemption as get_messages_as_conversation) and the
IN-clause pattern already used by _fetch_conversation_rows — and passes
include_ancestors=True at the REST messages endpoint and the gateway
API server's history loader.
After a compression rotation the transcript spans parent sessions; the
REST endpoint previously returned only the child continuation, hiding
the pre-compaction transcript while session.resume showed it (#51058).
Tests: 3 new tests in tests/test_hermes_state.py (lineage expansion,
explicit-branch exemption, merged-set paging); 34/34 lineage/branch/resume
suite green.
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