This PR replaces raw PinnedSpanByte arrays and manual pointer management in transaction cluster slot verification and WATCH key storage with SessionParseState and ScratchBufferAllocator.

Problem:

• Cluster key tracking: TransactionManager stored keys for slot verification as raw PinnedSpanByte pointers into the network receive buffer. Because the receive buffer can be reallocated between MULTI and EXEC, a fragile UpdateRecvBufferPtr method was needed to walk all stored pointers and rebase them — prone to dangling pointer bugs.
• Inconsistent slot verification: EXEC used NetworkKeyArraySlotVerify (iterating Span<PinnedSpanByte>), while all other multi-key slot verification paths already use the SessionParseState-based NetworkMultiKeySlotVerify overload.
• Watch key storage: WatchedKeysContainer managed its own raw byte[] buffer with manual pointer arithmetic and pointer fixup on resize — duplicating logic that ScratchBufferAllocator already provides.
• Missing watchContainer.Reset() in DISCARD and aborted EXEC paths left watched key slices pointing into invalidated scratch buffer memory (use-after-reset).

Changes introduced:

• Cluster key tracking:
  • Replaced PinnedSpanByte[] keys + keyCount with a SessionParseState clusterKeyParseState on TransactionManager
  • Keys are stored as raw pointers into the receive buffer (zero-copy fast path); only copied into a dedicated ScratchBufferAllocator (txnScratchBuffer) lazily when a receive buffer reallocation is detected
  • Eliminated UpdateRecvBufferPtr; replaced with CopyExistingKeysToScratchBuffer triggered by saveKeyRecvBufferPtr change detection
  • GetSlotVerificationInput builds ClusterSlotVerificationInput directly; EXEC uses the standard NetworkMultiKeySlotVerify(ref SessionParseState, ...) path
  • Added Capacity property to SessionParseState to support doubling growth
  • Added guard to skip slot verification when clusterKeyParseState.Count is 0 (e.g., MULTI/EXEC with only non-key commands)
• Watch key storage:
  • Replaced manual byte[] buffer + raw pointer arithmetic + pointer fixup on resize with a shared ScratchBufferAllocator (txnScratchBuffer)
  • Removed 5 fields: watchBuffer, watchBufferPtr, watchBufferHeadAddress, watchBufferSize, initialWatchBufferSize
  • Added watchContainer.Reset() to DISCARD and aborted EXEC paths to prevent use-after-reset of watched key slices
• Cleanup:
  • Removed unused NetworkKeyArraySlotVerify from RespServerSession, IClusterSession, and ClusterSession, along with its Span<PinnedSpanByte> MultiKeySlotVerify overload