prepare-sync: warn on close-cleanup failure (codeql)

PR-C.3 review fixup. The ``except Exception: pass`` after the
best-effort ``close_method()`` call tripped CodeQL's
``py/empty-except`` rule on two surfaces (code-quality + advanced
security). Cleanup is intentionally best-effort — a raise here
MUST NOT propagate or break sibling observers' dispatch — but
swallowing silently makes the rare cleanup-failure case
invisible.

Replace the empty pass with ``except Exception as close_error:``
followed by a ``warnings.warn`` mentioning the cleanup-failure.
Same isolation contract preserved (no propagation, no
sibling-blocking) but the swallow is now observable. CodeQL
``py/empty-except`` cleared on both surfaces.

Author: chris-colinsky

src/openarmature/graph/observer.py

@@ -423,8 +423,17 @@ def _dispatch(context: _InvocationContext, event: NodeEvent) -> None:
                 if close_method is not None:
                     try:
                         close_method()
-                    except Exception:
-                        pass
+                    except Exception as close_error:
+                        # Cleanup is best-effort: a raise here MUST NOT
+                        # propagate or block sibling observers. Surface
+                        # via ``warnings.warn`` so the swallow is at
+                        # least observable if it ever fires (CodeQL
+                        # py/empty-except clears on this surface too).
+                        warnings.warn(
+                            f"observer prepare_sync close cleanup raised "
+                            f"{type(close_error).__name__}: {close_error}",
+                            stacklevel=2,
+                        )
                 warnings.warn(
                     "observer prepare_sync returned an awaitable; "
                     "prepare_sync MUST be sync (define as `def`, not "