[codex] Isolate available plans Stripe imports

[tianmind-studio]

Summary

• Stub the Stripe module inside test_available_plans_resilience.py so the available-plans tests collect without the optional Stripe SDK installed.
• Restore the real backend/utils package path before importing routers.payment, while still stubbing heavy BYOK/external modules used only as import dependencies.
• Keep the tests exercising the real get_available_plans_endpoint behavior with explicit per-test Stripe method mocks.

Why

On a lightweight Windows backend test environment, collecting this test file failed at import stripe. When another lightweight test ran first, it could also leave a stubbed utils package with no filesystem path, causing utils.subscription import failures. This PR makes the test self-contained and order-tolerant without changing production code.

Validation

• python -m pytest backend\tests\unit\test_available_plans_resilience.py --collect-only -q --tb=short
• python -m pytest backend\tests\unit\test_available_plans_resilience.py -q --tb=short
• python -m pytest backend\tests\unit\test_action_item_date_validation.py backend\tests\unit\test_available_plans_resilience.py -q --tb=short
• python -m pytest backend\tests\unit\test_available_plans_resilience.py backend\tests\unit\test_action_item_date_validation.py -q --tb=short
• python -m black --line-length 120 --skip-string-normalization backend\tests\unit\test_available_plans_resilience.py
• python -m py_compile backend\tests\unit\test_available_plans_resilience.py
• git diff --check
• scripts\pre-commit