Close core runtime branch coverage gaps #414

Coverage review shows meaningful gaps in core runtime APIs, especially branch and result-path coverage.

Representative gaps from source-only local report:

• src/PatternKit.Core/Cloud/QueueLoadLeveling/QueueLoadLevelingPolicy.cs: 13 uncovered lines, 86.7%
• src/PatternKit.Core/Messaging/Consumers/EventDrivenConsumer.cs: 12 uncovered lines, 74.5%
• src/PatternKit.Core/Messaging/Consumers/EventDrivenConsumerHandlerResult and related result types have low branch coverage
• src/PatternKit.Core/Application/TableDataGateway result/gateway paths are below target
• src/PatternKit.Core/Cloud/Bulkhead, CacheAside, and GatewayAggregation still have branch gaps

Acceptance criteria:

• Add TinyBDD scenarios for failure, cancellation, empty, duplicate, invalid, timeout, and fallback paths.
• Target 95%+ for each touched core pattern class.
• Avoid testing implementation trivia; focus on public behavior and edge contracts.