Add edge case tests for devices field in loadConfig()

[groupsky]

Problem

The test coverage for the devices field in loadConfig() only covers the happy path (non-empty array). Missing critical edge cases that the CLI depends on.

Discovered during aspected review of #347 fix.

Current Coverage

Test added in #347 (config.test.ts lines 121-141):

• ✅ Non-empty devices array with valid device

Missing Edge Cases

1. Empty Devices Array

Why it matters: CLI explicitly checks for config.devices.length > 0 (cli.ts line 103)

Test needed:

it('should preserve empty devices array in config', async () => {
  const configJson = JSON.stringify({
    mqtt: { url: 'mqtt://localhost:1883' },
    devices: [],
  })

  mockedReadFile.mockResolvedValue(configJson)
  const config = await loadConfig('/path/to/config.json')

  expect(config.devices).toBeDefined()
  expect(config.devices).toEqual([])
})

2. Omitted Devices Field

Why it matters: Ensures backward compatibility for configs without device support

Test needed:

it('should not include devices field when omitted from config', async () => {
  const configJson = JSON.stringify({
    mqtt: { url: 'mqtt://localhost:1883' },
  })

  mockedReadFile.mockResolvedValue(configJson)
  const config = await loadConfig('/path/to/config.json')

  expect(config.devices).toBeUndefined()
  expect('devices' in config).toBe(false)
})

3. Invalid Device Configuration

Why it matters: Zod schema should validate and reject invalid device configs

Test needed:

it('should throw error for invalid device configuration', async () => {
  const configJson = JSON.stringify({
    mqtt: { url: 'mqtt://localhost:1883' },
    devices: [
      {
        deviceId: 'device+invalid',  // Contains MQTT special character
        driver: '@ya-modbus/driver-ex9em',
        connection: { type: 'rtu', port: '/dev/ttyUSB0', baudRate: 9600, slaveId: 1 },
      },
    ],
  })

  mockedReadFile.mockResolvedValue(configJson)

  await expect(loadConfig('/path/to/config.json')).rejects.toThrow('Invalid configuration')
})

Impact

Without these tests:

• Edge cases could break without detection
• CLI behavior depends on these cases (empty array vs undefined)
• No regression protection for validation logic

Files Affected

• packages/mqtt-bridge/src/utils/config.test.ts

Related

• loadConfig() does not load devices from configuration #347 - Original fix added basic test
• Aspected review identified gaps in coverage

[groupsky]

✅ VERIFIED FIXED: config-loader.test.ts (lines 217-283) has comprehensive edge case tests for devices field including: missing devices config, empty devices array (explicitly tests devices: [] and expects 'At least one device must be configured' error), and non-array devices validation.