fix(cli): remove dead peer turn session helpers and their test-only callers

[1688mengdie]

Summary

Fixes #2569

Delete the dead session-scoped turn helpers in src/apps/cli/src/peer_host/state.rs together with their test-only callers:

• PeerTurnTracker::drain_session_turns (zero non-test references left)
• session_tree_keys (private helper of the above)
• remove_completed_background_sources_for_session (private helper of the above)
• the four unit 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

After the upstream transactional session rollback rework, session-level turn cleanup moved into the Agent Runtime rollback path; the CI logs report all three symbols as dead code (see issue #2569 for the run evidence with file:line and rule names).

Type and Areas

Type: bug fix (dead code removal, no behavior change)

Areas: Rust CLI

Motivation / Impact

No direct user-facing change. Live peer-turn functionality is untouched: PeerTurnDrain, interrupt_event_stream, drain_peer_turns, peer_turns_for_cancellation and all remaining peer-host tests stay. The crate stops carrying three dead symbols and four orphaned tests, and the CI logs stop reporting the three dead_code warnings on every full run.

Verification

• cargo check --locked -p bitfun-cli on Windows: exit 0, the three dead_code warnings gone.
• cargo test --locked -p bitfun-cli --bin bitfun peer_host on Windows: 77 passed, 0 failed (the remaining peer-host test suite is intact after the four drain-only tests were removed).
• Full cross-platform test surface depends on the CI ubuntu/macos runners: the CLI Tests jobs should confirm the removal compiles clean on all platforms (n/a locally — no Linux/macOS machine available; the platform matrix in CI is exactly the place this gets verified).

Reviewer Notes

• Single file, −171 lines, pure deletion (0 additions): no new code, no #[allow(dead_code)] suppression.
• Deletion rationale is grep-based (zero production callers) plus the upstream CI dead_code logs; removal of the four tests is exactly scoped to the ones that exercise only the deleted entry point.
• AI-assisted change, lightly tested (cargo check + targeted unit tests on Windows; cross-platform behavior verified by CI).

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable. (n/a — no user-facing change)