mi: correctness fixes for unmarshal, resource leak, and test robustness (#2501)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

internal/mi/char16_test.go

@@ -0,0 +1,73 @@
+// SPDX-License-Identifier: Apache-2.0
+//
+// Copyright The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//go:build windows
+
+package mi
+
+import (
+	"testing"
+)
+
+// TestGetValue_CHAR16 verifies that GetValue treats CHAR16 as a scalar
+// (returning the code-unit directly as uint16) rather than dereferencing
+// element.value as a string pointer, which would crash.
+func TestGetValue_CHAR16(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		code uintptr
+		want uint16
+	}{
+		{"letter_A", 'A', 'A'},
+		{"digit_0", '0', '0'},
+		{"space", ' ', ' '},
+		{"euro_sign", 0x20AC, 0x20AC},
+		{"null", 0, 0},
+		{"max_bmp", 0xFFFF, 0xFFFF},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			e := Element{
+				value:     tt.code,
+				valueType: ValueTypeCHAR16,
+			}
+
+			got, err := e.GetValue()
+			if err != nil {
+				t.Fatalf("GetValue() error: %v", err)
+			}
+
+			v, ok := got.(uint16)
+			if !ok {
+				t.Fatalf("GetValue() returned %T, want uint16", got)
+			}
+
+			if v != tt.want {
+				t.Errorf("GetValue() = 0x%04X (%c), want 0x%04X (%c)", v, v, tt.want, tt.want)
+			}
+		})
+	}
+}
+
+// Note: The CHAR16 unmarshal branch in unmarshalInstance (session.go) handles
+// string and uint fields. Integration testing of this path requires a WMI class
+// with a CHAR16 property, which is extremely rare. The setUintField and
+// setIntField helpers used by the branch are covered by the signed/unsigned
+// integer tests (Test_MI_QueryUnmarshal_SignedInt, Test_MI_Unmarshal_TypeMismatch).