test(integration): poll subaccount fixture for eventual consistency

[TexasCoding]

Summary

`POST /portfolio/subaccounts` returns the new `subaccount_number` immediately, but `GET /portfolio/subaccounts/balances` lags by ~1.0-1.5s before the new account appears in the list. The `ephemeral_subaccount` fixture asserted visibility immediately after create, so it failed when the listing hadn't propagated yet.

Probe evidence:

```
created: 23
poll 1 (0.0s): created 23 in list? False
poll 2 (0.5s): created 23 in list? False
poll 3 (1.0s): created 23 in list? True
```

Replaced the immediate return with a 10s poll loop on `list_balances` (0.5s interval). Soft-skips with a clear message if the new number never appears within the budget.

Verification

• `tests/integration/test_subaccounts.py` — 13 passed, 1 skipped (skip is the unrelated `integration_real_api_only` netting endpoint)
• Previously: `test_list_balances_reflects_ephemeral_subaccount` was the only failure here

Progress on #124

Closes 1 of 11. Running tally: A (#126=4) + B (#127=1) + E (this=1) = 6 of 11. Remaining 5 are the `not_found` cluster in test_order_groups and test_orders (likely demo limitations or test setup issue — investigating in the next PR).

Refs #124

Test plan

• Local integration suite for test_subaccounts.py passes
• CI green

[claude]

Code Review

Overview

This PR fixes a legitimate eventual-consistency race in the ephemeral_subaccount session-scoped fixture. After POST /portfolio/subaccounts, the balances listing lags by ~1.0–1.5s on demo, causing test_list_balances_reflects_ephemeral_subaccount to fail sporadically. The fix polls list_balances for up to 10s (0.5s interval) and soft-skips if the account never surfaces.

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✅ What's done well

• Clock choice is correct. time.monotonic() is the right choice here — it's unaffected by system clock adjustments, unlike time.time().
• Surgical change. Only the fixture body is touched. No unrelated cleanup.
• pytest.skip semantics are correct. Called from a session-scoped fixture, it propagates to all dependent tests — appropriate for a demo-side propagation failure that's outside the test's control.
• Field access is consistent. balances.subaccount_balances matches the field name used in test_list_balances and test_list_balances_reflects_ephemeral_subaccount.
• Import ordering. import time is inserted alphabetically between logging and uuid. ✓
• 10s budget is defensively generous. Observed lag is 1.0–1.5s; 20 attempts gives a wide margin for slow CI environments.
• Docstring update is accurate and explains the why, not just the what.

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Suggestions

1. Add a log line on successful poll (minor)

The fixture already logs at create time. Logging when the account becomes visible makes CI output easier to parse when investigating future failures:

if any(b.subaccount_number == created for b in balances.subaccount_balances):
    elapsed = time.monotonic() - (deadline - 10.0)
    logger.info("Subaccount %s visible in list_balances after %.1fs", created, elapsed)
    return created

Not a blocker — just adds observability in the same style as the existing logger.info.

2. First poll always misses (cosmetic, not a bug)

Given the ~1.0–1.5s observed lag, the first two polls (at t≈0s and t≈0.5s) will always fail before the third one (t≈1.0s) succeeds. An optional micro-optimization would be an initial time.sleep(0.8) before entering the loop to skip two guaranteed misses. Whether this is worth the added complexity is a judgment call — the current approach is simpler and perfectly correct.

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Potential concerns

None that would block merging. The change is minimal, correct, and well-motivated. The pytest.skip path (10s timeout) is a last resort for demo-side failures unrelated to the SDK itself.

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Verdict

LGTM. Clean, minimal fix for a legitimate flaky test root cause. The polling strategy is sound, the fallback is appropriate, and the change touches nothing beyond what's necessary.