[mathcore] Fix TKDTreeBinning::FindBin returning out-of-range bins

TKDTreeBinning computed fNBins independently from the kd-tree it builds.
SetNBins picks a bucket size of fDataSize / nBins (integer division), but
the underlying TKDTree creates ceil(fNPoints / bucketSize) terminal nodes.
When the requested number of bins does not divide the data size evenly, the
bucket size is rounded down and the tree ends up with more terminal nodes
than fNBins. Since FindBin returns a terminal-node index, it could return a
value >= GetNBins(), pointing at a bin with no edges and no content:

```c++
binning.GetNBins()                         # 1001
binning.FindBin([1.2, 0.5, 0.5, 0.5, 0.5]) # 1004
binning.GetBinContent(1004)                # RuntimeWarning: No such bin
```

Set fNBins to the actual number of terminal nodes (GetNNodes() + 1) after
building the tree, so FindBin always returns an index within
[0, GetNBins()) and every bin has valid edges and content. This also
self-corrects existing objects on read-back, as the Streamer rebuilds the
tree via SetNBins.

Also fix SetBinsContent to read the per-node point counts directly from the
tree (GetNPointsNode) instead of an incorrect last-bin heuristic, so bin
contents are exact and sum to the data size.

Add a regression test covering the non-evenly-dividing case.

Closes #10786.

🤖 Done with the help of AI.

Author: guitargeek

math/mathcore/src/TKDTreeBinning.cxx

@@ -136,6 +136,9 @@ void TKDTreeBinning::SetNBins(UInt_t bins) {
          fDataBins = new TKDTreeID(fDataSize, fDim, fDataSize / (fNBins - remainingData)); // TKDTree input is data size, data dimension and the content size of bins ("bucket" size)
          SetTreeData();
          fDataBins->Build();
+         // Need to keep the number of bins consistent with the number of
+         // terminal nodes in the kd-tree.
+         fNBins = fDataBins->GetNNodes() + 1;
          SetBinsEdges();
          SetBinsContent();
       } else {
@@ -238,12 +241,13 @@ void TKDTreeBinning::SetTreeData() {
 }
 
 void TKDTreeBinning::SetBinsContent() {
-   // Sets the bins' content
+   // Sets the bins' content from the actual number of points in each terminal
+   // node of the kd-tree. All terminal nodes hold fBucketSize points except
+   // possibly the last one, so query the tree directly instead of guessing.
    fBinsContent.resize(fNBins);
+   UInt_t nTreeNodes = fDataBins->GetNNodes();
    for (UInt_t i = 0; i < fNBins; ++i)
-      fBinsContent[i] = fDataBins->GetBucketSize();
-   if ( fDataSize % fNBins != 0 )
-      fBinsContent[fNBins - 1] = fDataSize % (fNBins-1);
+      fBinsContent[i] = fDataBins->GetNPointsNode(i + nTreeNodes);
 }
 
 void TKDTreeBinning::SetBinsEdges() {