fix(brokers): correct rh_best_bid_ask to what the venue actually sends

[eaitbrahim]

Closes the fixture half of #413.

The wrong claim

tests/fixtures/rh_best_bid_ask.json carried:

{"next": null, "previous": null,
 "results": [{"symbol": "BTC-USD", "timestamp": "...", "bid": "65380.00", "ask": "65480.00"}]}

A tidy 15 bps spread BTC-USD does not produce, and next/previous this endpoint does not send.

What the venue actually returns

Raw body, verbatim:

{"results":[{"symbol":"BTC-USD","timestamp":"2026-08-19T19:20:41.579217945Z","bid":"68329.2","ask":"68324.17"}]}

Three samples ~2s apart, five pairs, 2026-08-19:

symbol	ask − bid ①	②	③
BTC-USD	−0.86 bps	−0.28 bps	−1.38 bps
ETH-USD	+0.10 bps	−0.29 bps	−1.29 bps
DOGE-USD	−0.11 bps	−0.11 bps	−0.59 bps
XLM-USD	+3.21 bps	+3.63 bps	+5.09 bps
ADA-USD	+2.41 bps	+2.41 bps	+2.49 bps

Not a label swap — that would invert every pair equally. The legs are sampled independently and then stamped with one timestamp the row does not earn: where the true spread is thinner than the sampling jitter (BTC, ETH, DOGE, all well under 1 bp) they land out of order; where it is wider (XLM, ADA) the ordering survives.

Why the probe never caught it

scripts/robinhood_smoke.py compares shapes, never values. It matched this fixture on every run. This is #217 F4's failure inverted — that fixture invented keys the venue never sends, this one invented an ordering it does not produce, which is harder to spot because a tidy spread is what a reader expects to see.

The fix

• rh_best_bid_ask.json — an observed crossed BTC-USD row, no next/previous.
• rh_best_bid_ask_uncrossed.json — new, an observed XLM-USD row.

Both, because either alone would be a fresh false claim: a lone crossed row asserts "always crossed" as confidently as the old one asserted "never". That is #230's lesson — a fixture can be wrong by being unrepresentative, not only by being invented.

Two tests asserted bid < ask, one per side of the port, and are inverted. The adapter-side test now pins that the endpoint admits both orderings, and that the crossing is jitter-sized (< 5 bps) rather than a semantic inversion — so a future row crossing by percent reads as the different, worse finding it would be.

transport.get_best_bid_ask gains the warning, including that translate.to_price_side's BUY→ask / SELL→bid mapping inverts on a crossed row (both directions optimistic — the one outcome that mapping exists to prevent). Its field list also claimed rows carry symbol, bid and ask while omitting timestamp, which #217 F8 had already established; corrected.

The README's "the five READ fixtures match observed responses" gains the #413 caveat next to the #230 one.

Scope

No behaviour change — nothing calls this endpoint today. Deciding the contract (normalise vs. declare it unusable for pricing) stays on #413, so a first consumer cannot pick silently.

Full suite: 3556 passed, 3 skipped. ruff check and mypy clean.

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