[Bug]: dead_code warnings for peer-host session turn drain helpers with no production callers

Summary

The upstream CI logs for main report three dead-code warnings in the peer host turn tracker. Evidence from GitHub Actions run 33035362609 (reproduced across runs 33050974141 and 33047041028):

• warning: method drain_session_turns is never used --> src/apps/cli/src/peer_host/state.rs:426:19
• warning: function session_tree_keys is never used --> src/apps/cli/src/peer_host/state.rs:662:4
• warning: function remove_completed_background_sources_for_session is never used --> src/apps/cli/src/peer_host/state.rs:737:4

Cross-checking the crate: PeerTurnTracker::drain_session_turns has zero non-test references, and session_tree_keys / remove_completed_background_sources_for_session are called only by drain_session_turns itself and by tests that exclusively exercise it. Four unit tests exist solely to exercise drain_session_turns.

Root Cause Analysis

• After the upstream transactional session rollback rework, session-level turn cleanup moved into the Agent Runtime rollback path. The old session-scoped drain entry point on PeerTurnTracker lost all production callers, and its two private helpers plus the four tests that only exercise it became dead weight.
• rustc therefore reports all three symbols as dead code on every full CI run of main (runs 33035362609 / 33050974141 / 33047041028).

Proposed Fix

Delete the three dead symbols (drain_session_turns, session_tree_keys, remove_completed_background_sources_for_session) together with the four tests that exclusively exercise drain_session_turns:

• draining_a_parent_session_after_root_completion_returns_the_active_child_only
• draining_a_child_session_releases_its_early_follow_up_reservation
• draining_a_sibling_child_does_not_release_another_childs_reservation
• explicit_drains_quarantine_removed_turn_ids

Live peer-turn functionality (PeerTurnDrain, interrupt_event_stream, drain_peer_turns, peer_turns_for_cancellation, and the remaining 70+ peer_host tests) is untouched. No #[allow(dead_code)] suppression is involved.

Start in src/apps/cli/src/peer_host/state.rs at the three reported symbols and inspect the four named tests that exclusively exercise drain_session_turns. Verify the remaining peer-turn tests still cover live functionality, then run the peer_host test suite and a full build or CI-equivalent check. Done means the obsolete symbols and tests are removed without dead-code warnings or regressions.