test: cover desc address ordering on preview accounts/:stake_address/addresses

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What

Adds an order=desc case for /accounts/{stake_address}/addresses to the preview fixture set. It reuses the account from the existing "bf stake address with many addresses" fixture.

Why

desc ordering for this endpoint was only covered by a mainnet fixture. Implementations that run the suite against preview never exercise it. This gap let an ordering bug in dolos/minibf go unnoticed (see txpipe/dolos#1140, fixed by txpipe/dolos#1221).

The account discriminates the common failure mode well. Its oldest address is also its most recently used one:

address	first use (block)	last use (block)
…surhhy9	317791	2207128
…snq24zy	318188	2206793
…srdg8p9	515922	515922

Blockfrost orders addresses by first on-chain appearance, and desc returns the exact reverse of the asc list: […srdg8p9, …snq24zy, …surhhy9]. An implementation that orders desc by latest appearance returns […surhhy9, …snq24zy, …srdg8p9] instead — maximally wrong, so the test cannot pass by accident.

Verification

• Expected response is the reverse of the already-verified asc fixture for the same account.
• Cross-checked first/last appearance blocks via Koios preview.
• update-fixtures-ids, check-fixture-wiring, type-check, lint, and unit tests all pass.
• I could not run the integration suite against production Blockfrost preview (no project id at hand) — please give it one CI run.

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