test(pr-cost): close the last two gaps from the superseded branch

[cheshirecode]

Why

Auditing whether fix/pr-cost-cache-visibility was safe to delete left two of its assertions unrepresented on main. This carries both, so nothing in that branch is unique any more and it can be deleted without losing coverage.

The two gaps

1. Only claude-opus-4-6 was asserted against the 3× overcharge.

claude-opus-4-7 and claude-opus-4-8 are separate table rows. A test covering one row leaves the other two free to be deleted back into the overcharging claude-opus-4 prefix — the exact defect #35 fixed. The test now covers all three via subTest.

2. The corrected-repost test asserted the row COUNT, not the figures.

self.assertEqual(len(rows), 2)   # two rows holding the SAME wrong number passes this

That is the whole point of --allow-duplicate: the second row is a correction. It now asserts [517.46, 602.99], and pins in the same place that the unflagged second call still reports "duplicate", so the idempotent default is covered too.

Both proved by mutation

Against scratchpad copies of the skill tree, so no tracked source was written:

Mutation	Result
Delete the 4-7 and 4-8 rate rows	fails test_opus_4_6_and_later_are_not_priced_as_opus_4_0
Write the correction as a copy of the row already in the ledger	fails test_allow_duplicate_publishes_a_corrected_figure

The second mutation is the interesting one: it still lands two ledger rows, so the old length check would have passed it while the correction was silently lost. That is what the value assertion buys.

Verification

tests/run.sh                                       -> 157 pass, 0 fail, exit 0
python3 -m unittest discover skills/pr-cost/tests  -> 81 tests OK
ruff check (both changed files)                    -> clean

Tests only — no behaviour change.