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).

agent/context_engine.py

@@ -78,6 +78,7 @@ class ContextEngine(ABC):
         self,
         messages: List[Dict[str, Any]],
         current_tokens: int = None,
+        focus_topic: str = None,
     ) -> List[Dict[str, Any]]:
         """Compact the message list and return the new message list.
 
@@ -86,6 +87,12 @@ class ContextEngine(ABC):
         context budget. The implementation is free to summarize, build a
         DAG, or do anything else — as long as the returned list is a valid
         OpenAI-format message sequence.
+
+        Args:
+            focus_topic: Optional topic string from manual ``/compress <focus>``.
+                Engines that support guided compression should prioritise
+                preserving information related to this topic.  Engines that
+                don't support it may simply ignore this argument.
         """
 
     # -- Optional: pre-flight check ----------------------------------------
@@ -98,6 +105,21 @@ class ContextEngine(ABC):
         """
         return False
 
+    # -- Optional: manual /compress preflight ------------------------------
+
+    def has_content_to_compress(self, messages: List[Dict[str, Any]]) -> bool:
+        """Quick check: is there anything in ``messages`` that can be compacted?
+
+        Used by the gateway ``/compress`` command as a preflight guard —
+        returning False lets the gateway report "nothing to compress yet"
+        without making an LLM call.
+
+        Default returns True (always attempt).  Engines with a cheap way
+        to introspect their own head/tail boundaries should override this
+        to return False when the transcript is still entirely protected.
+        """
+        return True
+
     # -- Optional: session lifecycle ---------------------------------------
 
     def on_session_start(self, session_id: str, **kwargs) -> None: