Fix red develop 0970756859a6

The merge lane found `develop` red at `0970756859a6c237c5a55bd6935afe9c236308a8` on its own: no queued PR merges until it is fixed. Failing (tests and gate stages): - `tests/billing/test_plan_routes.py::test_no_amount_but_the_price_mirror_can_feed_a_plan_response` - `api-unit-dc-test` The whole gate log: artifact `13139` on lane task 11872. Make `develop` pass its full gate again. Open a PR as usual: the merge lane merges a PR that fixes some of these and adds no failure even while `develop` is red.

Fixed the price-mirror AST guard, which went red after Task 11865 moved the reads into entity_shape.pick: it now counts pick() string arguments as reads. test_plan_routes 64/64 and quick gate green. PR #7498 queued to the merge lane. Memory: none applied.