Harden diagnostic handling and warning presentation

internal/framework/contracts/diagnostics_test.go

@@ -146,6 +146,47 @@ func TestCloneDiagnosticCollectionOwnsGroupsAndChunkIndex(t *testing.T) {
 	}
 }
 
+func TestProjectDiagnosticCollectionPartitionsAndChecksTotals(t *testing.T) {
+	warning := validDiagnosticGroup(DiagnosticDispositionWarning, DiagnosticCategoryFallback, "fallback", 2)
+	diagnostic := validDiagnosticGroup(DiagnosticDispositionAdvisory, DiagnosticCategoryDataQuality, "quality", 3)
+	collection := DiagnosticCollection{
+		Groups:                       []DiagnosticGroup{warning, diagnostic},
+		Truncated:                    true,
+		UnrepresentedOccurrenceCount: 4,
+	}
+	projection, err := ProjectDiagnosticCollection(collection)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(projection.Warnings) != 1 || len(projection.Diagnostics) != 1 || projection.WarningOccurrenceCount != 2 || projection.DiagnosticOccurrenceCount != 7 {
+		t.Fatalf("projection = %#v, want partitioned exact totals", projection)
+	}
+	collection.Groups[0].Samples[0].Message = "mutated"
+	if projection.Warnings[0].Samples[0].Message != "message" {
+		t.Fatal("projection retained caller-owned sample storage")
+	}
+
+	overflow := DiagnosticCollection{Groups: []DiagnosticGroup{
+		validDiagnosticGroup(DiagnosticDispositionWarning, DiagnosticCategoryFallback, "first", int(^uint(0)>>1)),
+		validDiagnosticGroup(DiagnosticDispositionWarning, DiagnosticCategoryFallback, "second", 1),
+	}}
+	if _, err := ProjectDiagnosticCollection(overflow); err == nil {
+		t.Fatal("ProjectDiagnosticCollection() overflow error = nil")
+	}
+}
+
+func validDiagnosticGroup(disposition DiagnosticDisposition, category DiagnosticCategory, reason string, occurrences int) DiagnosticGroup {
+	return DiagnosticGroup{
+		Disposition:        disposition,
+		Category:           category,
+		ReasonCode:         reason,
+		Origin:             DiagnosticOrigin{Stage: DiagnosticOriginStageNormalize, StepID: "step", LaneID: "lane", ModuleKey: "module"},
+		OccurrenceCount:    occurrences,
+		Samples:            []DiagnosticSample{{Scope: "scope", Message: "message"}},
+		OmittedSampleCount: occurrences - 1,
+	}
+}
+
 func validProducerDiagnostic() ProducerDiagnostic {
 	return ProducerDiagnostic{
 		Disposition:     DiagnosticDispositionWarning,