test(perps): skip reduce-only-without-position test on external liquidity

`test_perp_reduce_only_rejected_without_position` rests a setup maker SELL at
oracle*1.04 to give the reduce-only IOC a counterparty. When the shared devnet
book has external liquidity (an MM bot quoting near pool price, ~+16% over
oracle), that maker SELL crosses immediately instead of resting, and settling
the resulting fill against the external counterparty currently 500s (off-chain
settlement-robustness bug, PRO-191) — so the test fails hard instead of skipping.

Add the same `skip_if_external_liquidity` guard its siblings already use (e.g.
`test_perp_ioc_taker_buy_matches_maker_sell`) so it self-skips on a dirty book.
Verified: the test now SKIPS cleanly against the live devnet book.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: arturbeg

tests/test_perps/test_limit_orders.py

@@ -194,6 +194,20 @@ async def test_perp_reduce_only_rejected_without_position(
         the test exists to catch
     """
     market_price = float(await perp_taker_tester.data.current_price(PERP_SYMBOL))
+
+    # Skip if an external MM is on the book: the setup maker SELL at oracle*1.04
+    # below would cross any external bid above that price and fill immediately
+    # instead of resting, so the reduce-only IOC would settle against external
+    # liquidity rather than exercise the zero-position on-chain check (and that
+    # settlement currently 500s). Mirrors `test_perp_ioc_taker_buy_matches_maker_sell`
+    # and the other controlled-book tests that self-skip on a dirty devnet book.
+    await skip_if_external_liquidity(
+        perp_taker_tester.data,
+        PERP_SYMBOL,
+        market_price,
+        reason_prefix="test_perp_reduce_only_rejected_without_position",
+    )
+
     await perp_taker_tester.check.position_not_open(PERP_SYMBOL)
 
     # Guarantee the IOC has a counterparty so the on-chain check actually