fix(python): call _history_error_envelope on the class

[Dusk1e]

Summary

_get_history_page calls _history_error_envelope(...) as a bare name, but f5f3bf8 moved that helper from module scope into BulkHttpClient as a @staticmethod. Class-body names are not in scope inside method bodies, so the call resolves against globals, finds nothing, and every non-2xx history response raises

NameError: name '_history_error_envelope' is not defined

instead of the intended HistoryHttpError. Callers that catch HistoryHttpError — the documented contract — never see it, and the status code and error envelope are lost.

All six public history readers go through _get_history_page, so all six are affected: get_fills_page, get_positions_page, get_funding_page, get_orders_page, get_activity_page, get_risk_page.

tests/test_history_http.py already covers the intended behaviour and fails on main:

$ python -m pytest tests/test_history_http.py -q
bulk_api/api/bulk_http.py:760: NameError

SUBFAILED(status=502) HistoryHttpTests::test_history_non_contract_errors_preserve_status_with_bounded_fallback
SUBFAILED(status=503) HistoryHttpTests::test_history_non_contract_errors_preserve_status_with_bounded_fallback
SUBFAILED(status=418) HistoryHttpTests::test_history_non_contract_errors_preserve_status_with_bounded_fallback
FAILED HistoryHttpTests::test_history_non_success_preserves_structured_status_and_body
4 failed, 13 passed, 26 subtests passed

Changes

Qualify the call with self. One line; no behaviour change on the success path, and the helper itself is unchanged.

Verification

$ python -m pytest tests/test_history_http.py -q
14 passed, 29 subtests passed

The existing tests already assert the parts that were unreachable — that the status code survives, that a well-formed {"error": {code, message}} body is preserved, and that an oversized or malformed body falls back to the HISTORY_HTTP_ERROR envelope.

I also checked the rest of bulk_api for the same pattern — a call to a bare name that only exists as a method on the enclosing class — and this was the only occurrence.