fix: prevent half-stopped camera state causing false ALREADY_STARTED.

[yujune]

✨ What's the context?

Fixes a “half‑stopped” state in MobileScannerPlugin where either device or captureSession could remain non-nil after stop/pause or failed start, causing subsequent start calls to throw ALREADY_STARTED_ERROR and potentially leaking resources.

For example, if captureSession is instantiated but device is not, the early return from guard let device... will cause unrelease of captureSession.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.35%. Comparing base (250bea4) to head (f1b04e8).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1547   +/-   ##
========================================
  Coverage    38.35%   38.35%           
========================================
  Files           42       42           
  Lines          996      996           
========================================
  Hits           382      382           
  Misses         614      614           

Flag	Coverage Δ
unittests	38.35% <ø> (ø)

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[juliansteenbakker]

Seems good to me!