fix: add comprehensive validation to DistributedTeacherModel constructor

The constructor was accessing _workers[0] without validating the array,
which could cause IndexOutOfRangeException or inconsistent behavior.

Added validation to fail fast in the constructor:
1. Check that workers array is non-empty (Length > 0)
2. Check that no worker is null
3. Check that all workers have the same OutputDimension

This prevents runtime errors when OutputDimension or GetLogits is invoked
and provides clear, actionable error messages indicating which validation failed.

Addresses code review comment from @coderabbitai.

Author: claude

src/KnowledgeDistillation/Teachers/DistributedTeacherModel.cs

@@ -18,6 +18,30 @@ public DistributedTeacherModel(
         AggregationMode aggregation = AggregationMode.Average)
     {
         _workers = workers ?? throw new ArgumentNullException(nameof(workers));
+
+        // Validate workers array is non-empty
+        if (_workers.Length == 0)
+            throw new ArgumentException("Workers array cannot be empty", nameof(workers));
+
+        // Validate no worker is null
+        for (int i = 0; i < _workers.Length; i++)
+        {
+            if (_workers[i] == null)
+                throw new ArgumentException($"Worker at index {i} is null", nameof(workers));
+        }
+
+        // Validate all workers have the same output dimension
+        int expectedOutputDim = _workers[0].OutputDimension;
+        for (int i = 1; i < _workers.Length; i++)
+        {
+            if (_workers[i].OutputDimension != expectedOutputDim)
+                throw new ArgumentException(
+                    $"Worker at index {i} has OutputDimension {_workers[i].OutputDimension}, " +
+                    $"but expected {expectedOutputDim} (from worker 0). " +
+                    $"All workers must have the same OutputDimension.",
+                    nameof(workers));
+        }
+
         _aggregation = aggregation;
     }