snake_case query params: send top_n / min_volume_usd

[martinkersner]

Summary

Aligns the Python SDK with the backend snake_case query-param migration (#7343 / PR #7356 / #7359, docs #7365). top_n / min_volume_usd are PERMANENT public params (data-api + MCP); the SDK was still sending the old camelCase keys.

Not an outage fix — backend dual-read still accepts camelCase. Canonical-alignment cleanup.

Changes

• liquidation.py heatmap() wire key topN → top_n.
• open_interest.py summary() wire key topN → top_n.
• _endpoints.py metadata keys topN → top_n (×2), minVolumeUsd → min_volume_usd (×2).

Design choice — Option A (keep public kwarg, snake_case only the wire key)

Kept the user-facing topN kwarg as-is and changed only the query-string key sent to the server. Zero breaking change; the issue explicitly allows user-facing names to stay. Renaming the kwarg would break existing callers for no behavioral gain. No SDK method exposes a minVolumeUsd kwarg, so only its metadata key needed renaming.

_endpoints.py codegen finding

_endpoints.py is generated (make python from openapi.yaml) — the generator/spec live upstream in the backend repo, not in this repo, so there is no local regen path here. The backend OpenAPI source (cmd/data-api/docs/openapi.yaml) already emits top_n / min_volume_usd as of #7365; the committed _endpoints.py was simply regenerated before that update and went stale. This is a manual edit that catches the SDK up — the next make python run reconciles cleanly (it will reproduce these same snake_case keys). I did not fabricate a codegen run.

Test plan

• Added tests/test_query_params.py — offline responses-mocked tests asserting heatmap/summary send top_n on the wire (and topN is absent).
• python -m pytest tests/test_query_params.py tests/test_api.py -v → 10 passed.
• flake8 datamaxi/datamaxi/liquidation.py datamaxi/datamaxi/open_interest.py datamaxi/_endpoints.py tests/test_query_params.py → clean.
• (Live tests in test_call.py / test_integration.py skip without DATAMAXI_API_KEY.)

Closes Bisonai/datamaxi-backend#7398