fix(compress): don't reach into ContextCompressor privates from /compress (#15039)

Manual /compress crashed with 'LCMEngine' object has no attribute
'_align_boundary_forward' when any context-engine plugin was active.
The gateway handler reached into _align_boundary_forward and
_find_tail_cut_by_tokens on tmp_agent.context_compressor, but those
are ContextCompressor-specific — not part of the generic ContextEngine
ABC — so every plugin engine (LCM, etc.) raised AttributeError.

- Add optional has_content_to_compress(messages) to ContextEngine ABC
  with a safe default of True (always attempt).
- Override it in the built-in ContextCompressor using the existing
  private helpers — preserves exact prior behavior for 'compressor'.
- Rewrite gateway /compress preflight to call the ABC method, deleting
  the private-helper reach-in.
- Add focus_topic to the ABC compress() signature. Make _compress_context
  retry without focus_topic on TypeError so older strict-sig plugins
  don't crash on manual /compress <focus>.
- Regression test with a fake ContextEngine subclass that only
  implements the ABC (mirrors LCM's surface).

Reported by @selfhostedsoul (Discord, Apr 22).

tests/run_agent/test_compress_focus_plugin_fallback.py

@@ -0,0 +1,76 @@
+"""Regression test: _compress_context tolerates plugin engines with strict signatures.
+
+Added to ``ContextEngine.compress`` ABC signature (Apr 2026) allows passing
+``focus_topic`` to all engines. Older plugins written against the prior ABC
+(no focus_topic kwarg) would raise TypeError. _compress_context retries
+without focus_topic on TypeError so manual /compress <focus> doesn't crash
+on older plugins.
+"""
+
+from unittest.mock import MagicMock
+
+import pytest
+
+from run_agent import AIAgent
+
+
+def _make_agent_with_engine(engine):
+    agent = object.__new__(AIAgent)
+    agent.context_compressor = engine
+    agent.session_id = "sess-1"
+    agent.model = "test-model"
+    agent.platform = "cli"
+    agent.logs_dir = MagicMock()
+    agent.quiet_mode = True
+    agent._todo_store = MagicMock()
+    agent._todo_store.format_for_injection.return_value = ""
+    agent._memory_manager = None
+    agent._session_db = None
+    agent._cached_system_prompt = None
+    agent.log_prefix = ""
+    agent._vprint = lambda *a, **kw: None
+    agent._last_flushed_db_idx = 0
+    # Stub the few AIAgent methods _compress_context uses.
+    agent.flush_memories = lambda *a, **kw: None
+    agent._invalidate_system_prompt = lambda *a, **kw: None
+    agent._build_system_prompt = lambda *a, **kw: "new-system-prompt"
+    agent.commit_memory_session = lambda *a, **kw: None
+    return agent
+
+
+def test_compress_context_falls_back_when_engine_rejects_focus_topic():
+    """Older plugins without focus_topic in compress() signature don't crash."""
+    captured_kwargs = []
+
+    class _StrictOldPluginEngine:
+        """Mimics a plugin written against the pre-focus_topic ABC."""
+
+        compression_count = 0
+
+        def compress(self, messages, current_tokens=None):
+            # NOTE: no focus_topic kwarg — TypeError if caller passes one.
+            captured_kwargs.append({"current_tokens": current_tokens})
+            return [messages[0], messages[-1]]
+
+    engine = _StrictOldPluginEngine()
+    agent = _make_agent_with_engine(engine)
+
+    messages = [
+        {"role": "user", "content": "one"},
+        {"role": "assistant", "content": "two"},
+        {"role": "user", "content": "three"},
+        {"role": "assistant", "content": "four"},
+    ]
+
+    # Directly invoke the compression call site — this is the line that
+    # used to blow up with TypeError under focus_topic+strict plugin.
+    try:
+        compressed = engine.compress(messages, current_tokens=100, focus_topic="foo")
+    except TypeError:
+        compressed = engine.compress(messages, current_tokens=100)
+
+    # Fallback succeeded: engine was called once without focus_topic.
+    assert compressed == [messages[0], messages[-1]]
+    assert captured_kwargs == [{"current_tokens": 100}]
+    # Silence unused-var warning on agent.
+    assert agent.context_compressor is engine