fix(compression): update _pre_msg_count after durable adoption

Update _pre_msg_count after adopting the durable transcript so the
post-compression log reflects the correct pre-adoption message count.
Also use the existing _live_child_id() helper in the updated test
instead of hand-rolled child-id extraction.

Follow-up to #72631.

agent/conversation_compression.py

@@ -1737,6 +1737,7 @@ def compress_context(
                         len(durable_parent),
                     )
                     messages = durable_parent
+                    _pre_msg_count = len(messages)
                     # Token estimate was for the stale snapshot; clear it so
                     # the compressor re-derives from the adopted transcript
                     # instead of under-counting the newly visible rows.

contributors/emails/kshitijkapoor0611@gmail.com

@@ -0,0 +1 @@
+kshitijk4poor

tests/agent/test_compression_concurrent_fork.py

@@ -377,9 +377,8 @@ def test_durable_message_committed_before_lease_is_adopted(
     assert returned is not stale_snapshot
     assert returned[0]["content"] == "[CONTEXT COMPACTION] summary"
     assert agent.session_id != parent_sid
-    child = db.find_live_compression_child(parent_sid)
-    assert child is not None
-    child_id = child["id"] if isinstance(child, dict) else child
+    child_id = _live_child_id(db, parent_sid)
+    assert child_id is not None
     assert child_id == agent.session_id
 
 def test_skipped_compression_returns_messages_unchanged(tmp_path: Path) -> None: