Add experimental IsolationForest trainer for ML.NET

[asp2286]

Related issues

Fixes #3043

Add Isolation Forest Anomaly Detection Trainer (Experimental)

This PR adds an Isolation Forest anomaly detection trainer for ML.NET.
Isolation Forest (Liu, Ting, Zhou, 2008) is a tree-ensemble algorithm for unsupervised anomaly detection that isolates outliers via random partitioning. It complements existing ML.NET anomaly detectors (e.g., SR-CNN, IID) with a density-agnostic approach.

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Motivation

• Provide a widely-used, general-purpose anomaly detection method.
• Works without strong distribution assumptions.
• Produces both a continuous anomaly score and a binary label.
• Achieves parity with popular libraries like scikit-learn.

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Design (v1, Experimental)

• Core engine: IsolationForestModel (pure C#) implements random partitioning trees, scoring, and SHAP-like path contributions.
• Pipeline integration: IsolationForestTrainer : IEstimator<ITransformer> appends:
  • Score (float, scaled 0–100; higher = more anomalous),
  • PredictedLabel (bool), thresholded by Contamination or explicit override.
• Options:
  • Trees
  • SampleSize (psi)
  • Seed
  • Contamination
  • ParallelBuild
  • ThresholdOverride

⚠️ Experimental note: v1 uses CustomMapping internally. Models trained with this trainer cannot currently be persisted with mlContext.Model.Save(). A follow-up will introduce a proper IsolationForestTransformer with save/load and efficient row-mapping.

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Usage

var pipeline = ml.Transforms.Concatenate("Features", "X1", "X2")
    .Append(new IsolationForestTrainer(new IsolationForestTrainer.Options
    {
        Trees = 200,
        SampleSize = 256,
        Contamination = 0.02
    }));

var model = pipeline.Fit(data);

[asp2286]

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[codecov]

Codecov Report

❌ Patch coverage is 87.38255% with 94 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.05%. Comparing base (fb39755) to head (adab91a).

Files with missing lines	Patch %	Lines
...crosoft.ML.IsolationForest/IsolationForestModel.cs	84.69%	30 Missing and 32 partials ⚠️
...osoft.ML.IsolationForest/IsolationForestTrainer.cs	77.86%	21 Missing and 8 partials ⚠️
...L.IsolationForest.Tests/IsolationForestAllTests.cs	98.56%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7497      +/-   ##
==========================================
+ Coverage   69.01%   69.05%   +0.03%     
==========================================
  Files        1482     1485       +3     
  Lines      273999   274744     +745     
  Branches    28258    28388     +130     
==========================================
+ Hits       189093   189717     +624     
- Misses      77520    77594      +74     
- Partials     7386     7433      +47     

Flag	Coverage Δ
Debug	69.05% <87.38%> (+0.03%)	⬆️
production	63.34% <83.02%> (+0.03%)	⬆️
test	89.49% <98.56%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...L.IsolationForest.Tests/IsolationForestAllTests.cs	98.56% <98.56%> (ø)
...osoft.ML.IsolationForest/IsolationForestTrainer.cs	77.86% <77.86%> (ø)
...crosoft.ML.IsolationForest/IsolationForestModel.cs	84.69% <84.69%> (ø)

... and 6 files with indirect coverage changes

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