fix(context): restore marker import in tests, drop unused compressor imports

The arg-truncation removal deleted the only uses of the marker constants in
agent/context_compressor.py (ruff F401), and the test module had been
importing _COMPRESSION_MARKER_PREFIX through it, so test_context_compressor
line 137 raised NameError. Import it from its home, agent.compression_marker.

agent/context_compressor.py

@@ -17,8 +17,6 @@ from typing import Any, Dict, List, Optional, Sequence, Tuple
 from agent.image_eviction_policy import outbound_image_retire_count
 from agent.compression_marker import (
     ELISION_MARKER_MAX_LEN,
-    _COMPRESSION_MARKER_PREFIX,
-    _COMPRESSION_MARKER_TEMPLATE,
     _elision_marker,
     elide,
     elide_middle,