test(device-profiler): improve JSON formatter integration tests

[groupsky]

Description

The CLI integration tests currently mock formatJSON, which means they test mock behavior rather than the actual JSON output.

Current Issue

In packages/device-profiler/src/cli.test.ts:

jest.mock('./json-formatter.js')

// In tests
const mockFormatJSON = jest.fn().mockReturnValue('{"test": "json"}')
;(jsonFormatterModule.formatJSON as jest.Mock) = mockFormatJSON

This tests that:

• The mock was called ✓
• console.log received the mock output ✓

But does NOT test:

• Actual JSON structure ✗
• Real formatJSON behavior ✗
• Integration between cli and formatter ✗

Proposed Solutions

Option A: Remove Mock (Recommended)

Test the actual integration:

// Remove jest.mock('./json-formatter.js')

it('should output JSON when format is json', async () => {
  // ... setup transport ...
  
  await runProfileScan({
    transport: mockTransport,
    type: RegisterType.Holding,
    startAddress: 0,
    endAddress: 0,
    format: 'json',
    port: '/dev/ttyUSB0',
  })

  const output = mockConsoleLog.mock.calls.find(call => 
    typeof call[0] === 'string' && call[0].includes('"timestamp"')
  )?.[0]
  
  const parsed = JSON.parse(output)
  expect(parsed).toMatchObject({
    timestamp: expect.any(String),
    scan: { type: 'holding', startAddress: 0, endAddress: 0 },
    connection: { port: '/dev/ttyUSB0' },
    results: expect.any(Array),
    summary: expect.any(Object)
  })
})

Option B: Add Snapshot Test

Keep mock but add real integration test:

it('should produce valid JSON output snapshot', async () => {
  // ... real integration test with snapshot
})

Option C: Document Separation

Keep mock and add comment explaining why:

// We mock formatJSON here to isolate CLI orchestration testing.
// The actual JSON structure is thoroughly tested in json-formatter.test.ts
jest.mock('./json-formatter.js')

Impact

• Option A: Better integration coverage, catches real bugs
• Option B: Both unit and integration coverage
• Option C: Current state is acceptable if documented

Recommendation

Option A - Remove mock for true integration testing. The json-formatter.test.ts already provides unit coverage.

Related

Identified in aspected review of #<PR_NUMBER>