fix: skip batch decode masks when unpadded

[anupsv]

Summary

• Return .none for unpadded single-token decode in BatchKVCache and BatchRotatingKVCache.
• Keep explicit array masks when any row has positive left padding.
• Add regression coverage for both cache types.

Why

BatchKVCache was forcing an explicit all-true mask for the dominant continuous-batching decode path. That differs from regular KVCache, which uses no mask for n == 1, and matches the Gemma 4 4-bit repetition failure mode reported upstream.

Tests

• swift test --filter ContinuousBatchingTests/testBatchedKVCachesSkipMaskForUnpaddedSingleTokenDecode (failed before fix, passes after)
• swift test --filter ContinuousBatchingTests/testBatchedKVCachesKeepMaskForPaddedSingleTokenDecode
• swift test --filter ContinuousBatchingTests