Guard _assert_error_span_extras against duplicate span names (#231)

Address CoPilot review on #230 (the guard landed after the squash-merge).
_assert_error_span_extras picked candidates[0] for a span name; with a
single-attempt fixture (124) names are unique, but a future fixture with
multiple same-named spans (e.g. call-level retry) could silently assert
the wrong span. Assert exactly one candidate so that case fails loudly
here instead, rather than duplicating _assert_span_tree_matches's full
name+attribute disambiguation.

Test-only; no behavior change.

Author: chris-colinsky

tests/conformance/test_observability.py

@@ -3906,6 +3906,11 @@ def _walk(entries: list[dict[str, Any]]) -> None:
             name = cast("str", entry["name"])
             candidates = by_name.get(name, [])
             assert candidates, f"expected a span named {name!r}; got {sorted(by_name)}"
+            assert len(candidates) == 1, (
+                f"_assert_error_span_extras assumes unique span names; {name!r} matched "
+                f"{len(candidates)}. Add name+attribute disambiguation if a fixture emits "
+                f"multiple same-named spans (e.g. call-level retry)."
+            )
             span = candidates[0]
             desc = entry.get("status_description")
             if desc is not None: