…3.8 hotfix #2)

Hotfix #2 for the parent_trace_id wire-add end-to-end work from
PR #64 / 0.13.7. PR #64 wired the field on the wire, but a
diagnostic script (sdk_diag.py) running on the deployed 0.13.7
revealed cost_events on the backend were missing parent_trace_id:

  trace_id=cccccccc-... parent_trace_id=NULL model=gpt-4.1-mini tokens=10

The drift was in _enrich_event's parent_trace_id fallback
(commit efff530): it used an "if not in enriched" guard, so when
langgraph.py::on_llm_end's _active_runs[run_id] lookup missed
(run_id drift between the auto-injected chat_model callback and
an explicit user-supplied one, or no matching on_llm_start because
the user wrapped the LLM call in a non-langgraph stack), the
field was absent, the trace_id fallback at line 2422 overwrote
the event with the chain contextvar, but parent_trace_id stayed
NULL because the old condition was skipped.

Override semantics: the chain contextvar is the single source of
truth for "what chain does this event belong to". Both
langgraph.py::on_llm_end's caller-set value and a non-langgraph
caller's absence resolve to the same contextvar; preferring the
contextvar when present is idempotent for the happy path AND
closes the drift in the unhappy path.

Also:
- Bump version 0.13.7 -> 0.13.8.
- Prepend v3.23 / 0.13.8 changelog entry.
- Rewrite the existing
  test_enrich_event_preserves_caller_set_parent_trace_id to
  match the new override semantics.
- Add 2 new regression tests in TestEnrichEventParentTraceOverride:
  test_enrich_event_sets_parent_trace_id_when_chain_contextvar_set
  (drift scenario) and
  test_enrich_event_parent_trace_id_matches_trace_id_in_chain_mode
  (SpanContext invariant).

Regression coverage: 24 tests in test_drift_fixes_2026_07_04.py
(22 prior + 2 new). Full critical suite 157/157 passed. ruff clean,
mypy 34/34 source files clean.

End-to-end after this hotfix: the next real LLM call inside a
chain contextvar should arrive at the backend with both
cost_events.trace_id and cost_events.parent_trace_id set to the
chain contextvar value, and the unified SELECT third JOIN arm
should populate the dashboard's Recent executions panel with
Model / Tokens / Cost on the orchestration row.