refactor: export SanitizeTail and DefaultLogTail directly instead of wrapping

- Rename sanitizeTail → SanitizeTail (exported)
- Rename defaultLogTail → DefaultLogTail (exported)
- Delete export_test.go (no longer needed)
- Update test to reference handlers.SanitizeTail/DefaultLogTail directly

internal/handlers/tail_validation_test.go

@@ -14,16 +14,16 @@ func TestSanitizeTail(t *testing.T) {
 		input    string
 		expected string
 	}{
-		{"empty uses default", "", handlers.ExportedDefaultLogTail},
+		{"empty uses default", "", handlers.DefaultLogTail},
 		{"valid small number", "50", "50"},
 		{"valid max boundary", "500", "500"},
 		{"exceeds max clamped", "501", "500"},
 		{"very large clamped", "999999", "500"},
-		{"non-numeric uses default", "abc", handlers.ExportedDefaultLogTail},
-		{"all keyword uses default", "all", handlers.ExportedDefaultLogTail},
-		{"negative uses default", "-1", handlers.ExportedDefaultLogTail},
-		{"zero uses default", "0", handlers.ExportedDefaultLogTail},
-		{"float uses default", "1.5", handlers.ExportedDefaultLogTail},
+		{"non-numeric uses default", "abc", handlers.DefaultLogTail},
+		{"all keyword uses default", "all", handlers.DefaultLogTail},
+		{"negative uses default", "-1", handlers.DefaultLogTail},
+		{"zero uses default", "0", handlers.DefaultLogTail},
+		{"float uses default", "1.5", handlers.DefaultLogTail},
 		{"one is valid", "1", "1"},
 	}
 
@@ -31,7 +31,7 @@ func TestSanitizeTail(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
 
-			got := handlers.ExportedSanitizeTail(tc.input)
+			got := handlers.SanitizeTail(tc.input)
 			if got != tc.expected {
 				t.Errorf("sanitizeTail(%q) = %q, want %q", tc.input, got, tc.expected)
 			}