fix(retain): preserve exception message in fact_extraction error summary (#2468)

`test_extraction_failure_at_retry_cap_fails_terminally` (added in #2418,
guarding the recovered-worker path from #2413) asserts that when fact
extraction fails terminally, the original exception message survives
into `async_operations.error_message` so an operator can tell apart a
structured-JSON parse failure from a rate-limit reset from a network
5xx — all of which can surface as the same exception types in different
code paths.

The formatter was joining only `type(err).__name__`, producing rows like
"chunk 0: RuntimeError". The exception message was discarded, leaving
worker failures unactionable and silently defeating the test. The test
ran for the first time on this branch (its original PR's test-api job
was skipped) and surfaced the bug.

Add the message to the summary: "chunk 0: RuntimeError: structured JSON
parse failed after all retain_extract_facts attempts". Same shape, just
the field the test was added to enforce.

Drive-by: pre-existing, unrelated to the include_entity_links work in
this PR — but the test is wired in now and CI won't go green without it.

Co-authored-by: Chris Latimer <chris.latimer@vectorize.io>

Author: nicoloboschi

hindsight-api-slim/hindsight_api/engine/retain/fact_extraction.py

@@ -1814,7 +1814,14 @@ async def extract_facts_from_text(
         total_usage = total_usage + chunk_usage
 
     if failed_chunks:
-        failed_summary = ", ".join(f"chunk {idx}: {type(err).__name__}" for idx, err in failed_chunks[:5])
+        # Include the exception message — not just the type — so operators
+        # can tell a structured-JSON parse failure apart from a rate limit
+        # apart from a network 5xx, all of which can surface as the same
+        # exception types. The error_message we propagate to the
+        # async_operations row is the only inspection surface a worker-side
+        # failure leaves behind, and a bare "chunk 0: RuntimeError" is not
+        # actionable.
+        failed_summary = ", ".join(f"chunk {idx}: {type(err).__name__}: {err}" for idx, err in failed_chunks[:5])
         quota_errors = [err for _, err in failed_chunks if isinstance(err, ProviderRateLimitResetError)]
         if quota_errors and len(quota_errors) == len(failed_chunks):
             retry_at = max(err.retry_at for err in quota_errors)