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_compressor.py

@@ -1099,6 +1099,21 @@ The user has requested that this compaction PRIORITISE preserving all informatio
 
         return max(cut_idx, head_end + 1)
 
+    # ------------------------------------------------------------------
+    # ContextEngine: manual /compress preflight
+    # ------------------------------------------------------------------
+
+    def has_content_to_compress(self, messages: List[Dict[str, Any]]) -> bool:
+        """Return True if there is a non-empty middle region to compact.
+
+        Overrides the ABC default so the gateway ``/compress`` guard can
+        skip the LLM call when the transcript is still entirely inside
+        the protected head/tail.
+        """
+        compress_start = self._align_boundary_forward(messages, self.protect_first_n)
+        compress_end = self._find_tail_cut_by_tokens(messages, compress_start)
+        return compress_start < compress_end
+
     # ------------------------------------------------------------------
     # Main compression entry point
     # ------------------------------------------------------------------