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).

run_agent.py

@@ -7556,7 +7556,12 @@ class AIAgent:
             except Exception:
                 pass
 
-        compressed = self.context_compressor.compress(messages, current_tokens=approx_tokens, focus_topic=focus_topic)
+        try:
+            compressed = self.context_compressor.compress(messages, current_tokens=approx_tokens, focus_topic=focus_topic)
+        except TypeError:
+            # Plugin context engine with strict signature that doesn't accept
+            # focus_topic — fall back to calling without it.
+            compressed = self.context_compressor.compress(messages, current_tokens=approx_tokens)
 
         todo_snapshot = self._todo_store.format_for_injection()
         if todo_snapshot: