test(providers): Mistral batch retry + retry-log detail + comment (#893 r4)

Round-4 review on PR #893 (no blockers, minor items):
- Document why Mistral's _is_transient is SDK-level only (429/5xx): a bare
  connection drop the SDK surfaces as httpx/ConnectionError isn't an SDKError
  and isn't retried here by design — the pod-rollover target is the gateway
  (OpenAI-compatible) path, which does cover connection errors.
- Include the last error (%r) in the retry helper's "not resolved after N
  attempts" error log.
- Add test_mistral_embed_batch_retries_on_5xx (batch path parity with embed()).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/providers/_retry.py

@@ -77,10 +77,11 @@ def retry_on_transient(
                         retry_delay = min(retry_delay * 2, MAX_RETRY_DELAY)
 
             logger.error(
-                "%s %s not resolved after %d attempts",
+                "%s %s not resolved after %d attempts: %r",
                 provider_name,
                 label,
                 MAX_RETRIES,
+                last_error,
             )
             if last_error is None:  # pragma: no cover — loop above always sets this
                 raise RuntimeError("retry loop exited without capturing an error")