Fixed answers that quietly skipped your data sources (#968)

app/Tests/Tools/ConfidenceThresholdTests.cs

@@ -0,0 +1,58 @@
+using AIStudio.Tools;
+
+namespace AIStudio.Tests.Tools;
+
+/// <summary>
+/// Checks that determining a confidence threshold survives a list with nothing in it.
+/// </summary>
+/// <remarks>
+/// GetConfidenceThreshold reaches for Min and Max, both of which throw on an empty sequence. Until
+/// v26.9.1 the only thing standing between them and that exception was a count check at each of the
+/// two call sites. The RAG process already lost such a guard once -- a log line was written before
+/// the check that was meant to protect it, which is how every new chat produced an
+/// InvalidOperationException nobody noticed, because the RAG process catches and logs everything.
+/// The threshold is now asked to defend itself, and these tests hold it to that.
+/// </remarks>
+[TestFixture]
+public sealed class ConfidenceThresholdTests
+{
+    private readonly record struct Decision(float Confidence) : IConfidence;
+
+    [Test]
+    public void AnEmptyListYieldsAThresholdInsteadOfThrowing()
+    {
+        IReadOnlyList<Decision> nothingWasDecided = [];
+        var targetWindow = new TargetWindow(1, 2, 3, 0f);
+
+        Assert.That(nothingWasDecided.GetConfidenceThreshold(targetWindow), Is.EqualTo(0f), "A threshold of zero keeps everything that follows, which is the harmless answer for a list that holds nothing to filter.");
+    }
+
+    [Test]
+    public void AnEmptyListIsAnsweredEvenWhenTheWindowDemandsItems()
+    {
+        //
+        // The window asks for between five and ten items while not a single one exists. The guard
+        // has to hold regardless of what the window wants:
+        //
+        IReadOnlyList<Decision> nothingWasDecided = [];
+        var demandingWindow = new TargetWindow(4, 5, 10, 0.5f);
+
+        Assert.That(nothingWasDecided.GetConfidenceThreshold(demandingWindow), Is.EqualTo(0f), "The number of items the window asks for cannot conjure items to measure.");
+    }
+
+    [Test]
+    public void ASpreadOfDecisionsIsNarrowedToTheTargetWindow()
+    {
+        //
+        // Guarding the empty case must not change what the threshold does with actual items. Two
+        // weak decisions and three strong ones, with a window asking for two to three:
+        //
+        IReadOnlyList<Decision> decisions = [new(0.1f), new(0.2f), new(0.9f), new(0.95f), new(1.0f)];
+        var targetWindow = new TargetWindow(1, 2, 3, 0f);
+
+        var threshold = decisions.GetConfidenceThreshold(targetWindow);
+        var survivors = decisions.Count(decision => decision.Confidence >= threshold);
+
+        Assert.That(survivors, Is.InRange(targetWindow.TargetWindowMin, targetWindow.TargetWindowMax), "The threshold exists to cut a list down to the size the window asks for, and the three strong decisions are what should be left.");
+    }
+}