refactor(compression): name the split-failure cooldown; drop duplicate tests

Review folds from the formal gate battery:
- _SPLIT_FAILURE_COOLDOWN_SECONDS = 60 replaces the bare literal, with a
  comment pinning WHY it is the timeout ladder's first rung (transient
  lease/DB condition) rather than the 600s summary-provider cooldown.
- publish_compression_child docstring now states the compression_lock_holder
  condition on the refresh guard.
- Dropped 2 of 3 extracted unit tests as duplicates of existing coverage in
  test_compression_rotation_state.py / test_context_compressor.py; kept the
  force-bypass test (only site pinning that behavior for split failures) and
  the E2E test (now asserting the named constant).

agent/conversation_compression.py

@@ -92,6 +92,13 @@ _TERMINAL_COMPRESSION_PROVENANCES = frozenset(
     }
 )
 
+# Cooldown armed when a compression SPLIT fails (session_split_failed /
+# rotation rollback, #97948 symptom B). Deliberately the FIRST rung of the
+# timeout ladder (60/300/900 in context_compressor.py), not the 600s
+# _SUMMARY_FAILURE_COOLDOWN_SECONDS: a split failure is usually a transient
+# lease/DB condition, unlike a persistent summary-provider fault.
+_SPLIT_FAILURE_COOLDOWN_SECONDS = 60
+
 # Stable marker the gateway matches on to re-tag the auto-compaction lifecycle
 # status as ``kind="compacting"`` (tui_gateway/server.py::_status_update), so
 # drivers like the desktop app can show an explicit "Summarizing…" indicator
@@ -5001,7 +5008,8 @@ def compress_context(
                 # a stub compressor must not mask the original error.
                 try:
                     agent.context_compressor._record_compression_failure_cooldown(
-                        60, f"session_split_failed: {e}",
+                        _SPLIT_FAILURE_COOLDOWN_SECONDS,
+                        f"session_split_failed: {e}",
                     )
                 except Exception:
                     logger.debug(

hermes_state.py

@@ -7072,8 +7072,9 @@ class SessionDB(SessionSearchMixin, SessionSchemaMixin, SessionPortabilityMixin)
         rows in ``(watermark, watermark_ceiling]`` are foreign concurrent
         tail. ``None`` = unbounded (no internal flush happened).
 
-        When *require_lease_refresh* is True, the lease is refreshed inside
-        the same transaction before the expiry check. This gives a refresher
+        When *require_lease_refresh* is True and *compression_lock_holder* is
+        set, the lease is refreshed inside the same transaction before the
+        expiry check. This gives a refresher
         that stopped due to transient DB failures one final chance to extend
         the lease, preventing wasted compression work. The refresh uses the
         same ``conn`` as the publication, so there is no TOCTOU window.

tests/agent/test_compression_concurrent_fork.py

@@ -2233,6 +2233,8 @@ def test_failed_split_arms_failure_cooldown(tmp_path: Path) -> None:
     """Regression #97948 symptom B: a failed split/archive must arm the
     compression failure cooldown so the next automatic turn cannot
     immediately re-run the identical doomed compression."""
+    from agent.conversation_compression import _SPLIT_FAILURE_COOLDOWN_SECONDS
+
     db = SessionDB(db_path=tmp_path / "state.db")
     session_id = "SPLIT_FAIL_COOLDOWN_TEST"
     db.create_session(session_id, source="test")
@@ -2255,5 +2257,5 @@ def test_failed_split_arms_failure_cooldown(tmp_path: Path) -> None:
         "split failure must arm the failure cooldown (#97948 symptom B)"
     )
     seconds, error = cooldown_calls[0].args
-    assert seconds == 60
+    assert seconds == _SPLIT_FAILURE_COOLDOWN_SECONDS
     assert "session_split_failed" in str(error)

tests/agent/test_compression_split_failure_cooldown.py

@@ -1,8 +1,10 @@
 """Regression tests: a failed compression split must arm the failure cooldown.
 
-Extracted verbatim from PR #98137's test file (author: vsd2807); the sibling
-timeout-reconciliation tests were not carried because that production path was
-not salvaged (blocking review on #98137).
+Extracted from PR #98137's test file (author: vsd2807); two sibling unit
+tests were dropped as duplicates of existing coverage in
+test_compression_rotation_state.py / test_context_compressor.py, and the
+timeout-reconciliation tests were not carried because that production path
+was not salvaged (blocking review on #98137).
 
 Issue #97948 symptom B: without the cooldown, the turn after a
 session_split_failed abort immediately re-runs the identical doomed
@@ -12,34 +14,6 @@ compression.
 import time
 from unittest.mock import MagicMock
 
-def test_split_failure_records_cooldown():
-    """After a session split failure, compression failure cooldown is recorded."""
-    from agent.context_compressor import ContextCompressor
-
-    compressor = ContextCompressor.__new__(ContextCompressor)
-    compressor._summary_failure_cooldown_until = 0.0
-    compressor._last_summary_error = None
-    compressor._cooldown_persist_failed = False
-    compressor._session_db = None
-    compressor._session_id = ""
-
-    compressor._record_compression_failure_cooldown(60, "session_split_failed: test")
-
-    assert compressor._summary_failure_cooldown_until > time.monotonic()
-    assert compressor._last_summary_error == "session_split_failed: test"
-
-
-def test_cooldown_blocks_automatic_compression():
-    from agent.context_compressor import ContextCompressor
-
-    compressor = ContextCompressor.__new__(ContextCompressor)
-    compressor._summary_failure_cooldown_until = time.monotonic() + 60.0
-    compressor._last_summary_error = "session_split_failed"
-    compressor._cooldown_persist_failed = False
-    compressor.quiet_mode = True
-
-    assert compressor._automatic_compression_blocked_locally() is True
-
 
 def test_manual_compress_bypasses_cooldown():
     """Manual /compress (force=True) bypasses cooldown — existing behavior preserved."""