Update resolver docstring, drop test double-end

PR review follow-ups:
- _resolve_llm_parent docstring now lists the per-instance fan-out
  dispatch fallback added in this PR.
- Drop the manual dispatch_span.end() in the new test: shutdown()
  drains fan_out_instance_spans and ends the span, so the manual end
  double-ended it.

Author: chris-colinsky

src/openarmature/observability/otel/observer.py

@@ -1752,8 +1752,8 @@ def _resolve_llm_parent(
         calling_branch_name: str | None,
     ) -> object:
         """Look up the calling node's span using the calling-node
-        identity, fall back through subgraph dispatch / invocation
-        span."""
+        identity, falling back through the per-instance fan-out dispatch
+        span, subgraph dispatch / detached root, and the invocation span."""
         # 1. Direct match on the calling node's ``_StackKey``.
         calling_key: _StackKey = (
             calling_namespace_prefix,

tests/unit/test_observability_otel.py

@@ -601,7 +601,8 @@ async def test_llm_span_parents_under_fan_out_instance_dispatch() -> None:
                 branch_name=None,
             )
         )
-        dispatch_span.end()
+        # dispatch_span is ended by observer.shutdown() below (it drains
+        # fan_out_instance_spans); ending it here too would double-end it.
     finally:
         _reset_invocation_id(token)