Add admin-only camera access tests for picture handler

[Copilot]

Added coverage for the new admin_only camera flag to verify admin vs normal user access over the HTTP API.

• New handler tests: Exercised /picture/<id>/current with admin_only enabled/disabled to assert 200 for admins (signature-auth) and 403 for normal users.
• Test scaffolding: Stubbed handler response and injected minimal config/cache state for deterministic access-control checks.

Example:

# admin-only camera allows admin via signed request, blocks normal user
path = '/picture/1/current?_username=admin'
url = f'{path}&_signature={compute_signature("GET", path, b"", "adm1n")}'
resp = self.fetch(url)          # 200
resp_user = self.fetch('/picture/1/current')  # 403 when @admin_only = True

Original prompt

Please add tests for the new admin_only flag added in #3245. Open the pull requests against the HEAD branch of #3245.

The tests shall cover access with the users admin and user, which are hardcoded in motionEye, with and without the admin-only flag. Apart of unit tests, additional tests with the web API on a locally running motionEye daemon would be a great addition.

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