fix: Handle StopIteration locally in rate limit functions

StopIteration cannot be re-raised through the rate_limit() generator
due to PEP 479 (converted to RuntimeError). Handle it locally by
failing open without logging, while keeping the allocation policy
handlers for StopIteration from other code paths.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phacops

snuba/state/rate_limit.py

@@ -277,8 +277,11 @@ def rate_limit_start_request(
                 concurrent = sum(next(pipe_results) for _ in range(rate_limit_shard_factor))
             else:
                 concurrent = 0
-        except (RedisTimeoutError, StopIteration):
+        except RedisTimeoutError:
             raise
+        except StopIteration:
+            # Unexpected pipeline result count — fail open without logging
+            return RateLimitStats(rate=-1, concurrent=-1)
         except Exception as ex:
             # if something goes wrong, we don't want to block the request,
             # set the values such that they pass under any limit
@@ -315,8 +318,10 @@ def rate_limit_finish_request(
                 pipe.zincrby(query_bucket, -float(max_query_duration_s), query_id)
                 pipe.expire(query_bucket, max_query_duration_s)
                 pipe.execute()
-    except (RedisTimeoutError, StopIteration):
+    except RedisTimeoutError:
         raise
+    except StopIteration:
+        pass
     except Exception as ex:
         logger.exception(ex)