Centralize BD_DEBUG logging into internal/debug package

- Created internal/debug package with Enabled(), Logf(), Printf()
- Added comprehensive unit tests for debug package
- Replaced 50+ scattered os.Getenv("BD_DEBUG") checks across 9 files
- Centralized debug logic for easier maintenance and testing
- All tests passing, behavior unchanged

Closes bd-fb95094c.5

internal/debug/debug_test.go

@@ -0,0 +1,139 @@
+package debug
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"testing"
+)
+
+func TestEnabled(t *testing.T) {
+	tests := []struct {
+		name     string
+		envValue string
+		want     bool
+	}{
+		{"enabled with value", "1", true},
+		{"enabled with any value", "true", true},
+		{"disabled when empty", "", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			oldEnabled := enabled
+			defer func() { enabled = oldEnabled }()
+
+			if tt.envValue != "" {
+				enabled = true
+			} else {
+				enabled = false
+			}
+
+			if got := Enabled(); got != tt.want {
+				t.Errorf("Enabled() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestLogf(t *testing.T) {
+	tests := []struct {
+		name       string
+		enabled    bool
+		format     string
+		args       []interface{}
+		wantOutput string
+	}{
+		{
+			name:       "outputs when enabled",
+			enabled:    true,
+			format:     "test message: %s\n",
+			args:       []interface{}{"hello"},
+			wantOutput: "test message: hello\n",
+		},
+		{
+			name:       "no output when disabled",
+			enabled:    false,
+			format:     "test message: %s\n",
+			args:       []interface{}{"hello"},
+			wantOutput: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			oldEnabled := enabled
+			oldStderr := os.Stderr
+			defer func() {
+				enabled = oldEnabled
+				os.Stderr = oldStderr
+			}()
+
+			enabled = tt.enabled
+
+			r, w, _ := os.Pipe()
+			os.Stderr = w
+
+			Logf(tt.format, tt.args...)
+
+			w.Close()
+			var buf bytes.Buffer
+			io.Copy(&buf, r)
+
+			if got := buf.String(); got != tt.wantOutput {
+				t.Errorf("Logf() output = %q, want %q", got, tt.wantOutput)
+			}
+		})
+	}
+}
+
+func TestPrintf(t *testing.T) {
+	tests := []struct {
+		name       string
+		enabled    bool
+		format     string
+		args       []interface{}
+		wantOutput string
+	}{
+		{
+			name:       "outputs when enabled",
+			enabled:    true,
+			format:     "debug: %d\n",
+			args:       []interface{}{42},
+			wantOutput: "debug: 42\n",
+		},
+		{
+			name:       "no output when disabled",
+			enabled:    false,
+			format:     "debug: %d\n",
+			args:       []interface{}{42},
+			wantOutput: "",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			oldEnabled := enabled
+			oldStdout := os.Stdout
+			defer func() {
+				enabled = oldEnabled
+				os.Stdout = oldStdout
+			}()
+
+			enabled = tt.enabled
+
+			r, w, _ := os.Pipe()
+			os.Stdout = w
+
+			Printf(tt.format, tt.args...)
+
+			w.Close()
+			var buf bytes.Buffer
+			io.Copy(&buf, r)
+
+			if got := buf.String(); got != tt.wantOutput {
+				t.Errorf("Printf() output = %q, want %q", got, tt.wantOutput)
+			}
+		})
+	}
+}