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).

gateway/run.py

@@ -7102,10 +7102,7 @@ class GatewayRunner:
                 tmp_agent._print_fn = lambda *a, **kw: None
 
                 compressor = tmp_agent.context_compressor
-                compress_start = compressor.protect_first_n
-                compress_start = compressor._align_boundary_forward(msgs, compress_start)
-                compress_end = compressor._find_tail_cut_by_tokens(msgs, compress_start)
-                if compress_start >= compress_end:
+                if not compressor.has_content_to_compress(msgs):
                     return "Nothing to compress yet (the transcript is still all protected context)."
 
                 loop = asyncio.get_running_loop()