Replace get_invalid_fields_errors mocks in the /new/ UI tests with factory setup

[jaredlockhart]

The /new/ UI tests patch NimbusExperiment.get_invalid_fields_errors at 22 sites (21 in nimbus_ui/tests/test_new_views.py, 1 class-level patcher in nimbus_ui/tests/test_new_forms.py). This is internal review validation, not external IO, so it should not be mocked. Most sites pass return_value={}, which fabricates "this delivery is review-valid" instead of building one.

Both entry conditions can be built directly with factories:

• Review-valid: create_with_lifecycle(Lifecycles.CREATED, ...) with firefox_min_version above MIN_REQUIRED_VERSION, is_rollout=True, a single branch, and channels set per application.
• Has review errors: the factory default firefox_min_version (100) is below MIN_REQUIRED_VERSION (105), so a plain factory rollout already fails validation. Leaving a required field unset also works.

Find the sites with grep -n '"get_invalid_fields_errors"'.

Acceptance criteria:

• No mock.patch.object(NimbusExperiment, "get_invalid_fields_errors", ...) remains in nimbus_ui/tests/.
• Each affected test sets up its review state with factories and still asserts the same behaviour.
• make check passes with 100% coverage.

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