Fix BDX block data pointer to track delegate buffer adjustments (#71940) (#72346)

* Fix BDX block data pointer to track delegate buffer adjustments

In OnAckReceived(), blockData.Data used blockBuf->Start() (the original
buffer start) while blockData.Length used buffer.size() (potentially
reduced by the delegate). If a CollectLog delegate implementation
adjusts the span's data pointer (e.g., via SubSpan), the data pointer
and length would be misaligned, potentially sending uninitialized memory.

Use buffer.data() to correctly track any adjustment the delegate made.



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(cherry picked from commit 32f1ce0)

Co-authored-by: Justin Wood <woody@apple.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Andrei Litvin <andy314@gmail.com>

Author: 5 people

src/app/clusters/diagnostic-logs-server/BDXDiagnosticLogsProvider.cpp

@@ -197,8 +197,8 @@ void BDXDiagnosticLogsProvider::OnAckReceived()
 
     // Prepare the BDX block to send to the requestor
     TransferSession::BlockData blockData;
-    blockData.Data   = blockBuf->Start();
-    blockData.Length = static_cast<size_t>(buffer.size());
+    blockData.Data   = buffer.data();
+    blockData.Length = buffer.size();
     blockData.IsEof  = isEndOfLog;
 
     err = mTransfer.PrepareBlock(blockData);