Add unit tests for MetricCalculatorLogic command building

[tctco]

Motivation

• Verify that MetricCalculatorLogic builds correct CLI commands for different metric types and flag combinations.
• Allow these unit tests to run in CI by stubbing out unavailable runtime GUI dependencies (qt and slicer).

Description

• Add localizer/src/tests/test_ui_metric_command_builder.py which injects localizer/lib on sys.path and provides a _load_metric_calculator_logic helper that stubs qt and slicer before importing MetricCalculatorLogic.
• Add test_build_command_uses_subcommand_cli_for_centaur to assert the centaur subcommand, presence of --input/--output, and handling of --manual-fov and --skip-normalization.
• Add test_build_command_fillstates_requires_tracer_and_no_suvr and test_build_suvr_command_does_not_reference_legacy_flag to validate tracer handling for Fill States and that suvr commands do not reference legacy flags.

Testing

• Ran pytest localizer/src/tests/test_ui_metric_command_builder.py and the tests completed successfully.

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Codex Task