fix: fixed the BoolPtrValue in internal flag pkg

- Fixed the BoolPtrValue type in the internal flag package by adding the
  IsBoolFlag() bool method to indicate to the command-line parser that
  the flag is a boolean flag.
- Added unit tests for the BoolPtrValue type.

PR: #56

internal/flag/boolptrvalue.go

@@ -34,3 +34,5 @@ func (b *BoolPtrValue) Set(value string) error {
 
 	return nil
 }
+
+func (b *BoolPtrValue) IsBoolFlag() bool { return true }

internal/flag/boolptrvalue_test.go

@@ -0,0 +1,65 @@
+package flag_test
+
+import (
+	"slices"
+	"testing"
+
+	internalFlag "codeflow.dananglin.me.uk/apollo/enbas/internal/flag"
+)
+
+func TestBoolPtrValue(t *testing.T) {
+	tests := []struct {
+		input string
+		want  bool
+	}{
+		{
+			input: "True",
+			want:  true,
+		},
+		{
+			input: "false",
+			want:  false,
+		},
+	}
+
+	value := internalFlag.NewBoolPtrValue()
+
+	for _, test := range slices.All(tests) {
+		if err := value.Set(test.input); err != nil {
+			t.Fatalf(
+				"Unable to parse %s as a BoolPtrValue: %v",
+				test.input,
+				err,
+			)
+		}
+
+		got := *value.Value
+
+		if got != test.want {
+			t.Errorf(
+				"Unexpected bool parsed from %s: want %t, got %t",
+				test.input,
+				test.want,
+				got,
+			)
+		} else {
+			t.Logf(
+				"Expected bool parsed from %s: got %t",
+				test.input,
+				got,
+			)
+		}
+	}
+}
+
+func TestNilBoolPtrValue(t *testing.T) {
+	value := internalFlag.NewBoolPtrValue()
+	want := "NOT SET"
+	got := value.String()
+
+	if got != want {
+		t.Errorf("Unexpected string returned from the nil value; want %s, got %s", want, got)
+	} else {
+		t.Logf("Expected string returned from the nil value; got %s", got)
+	}
+}