fix(premium): snake_case source/target exchange+quote wire keys

[martinkersner]

Closes #119 (partial: Step 1 only)

Summary

premium.__call__ hand-translated 4 snake_case Python kwargs into camelCase wire keys, which the snake_case backend silently ignored — so source/target exchange + quote filtering was broken against the live API. Fixed the 4 wire keys to snake_case:

• sourceExchange -> source_exchange
• targetExchange -> target_exchange
• sourceQuote -> source_quote
• targetQuote -> target_quote

Public Python kwargs were already snake_case; only the wire keys were wrong. All other params already used snake_case keys and are untouched. self.query("/api/v1/premium", params) left in place.

Deferred (NOT in this PR)

Step 2 — expanding the codegen registry (datamaxi/_endpoints.py, op premium) in the upstream ../datamaxi-backend OpenAPI spec, live param-by-param verification, converting premium.__call__ onto request_endpoint, and adding tests/test_integration.py premium cases — is UPSTREAM-BLOCKED and intentionally deferred. The registry is incomplete for premium, so routing through request_endpoint (which rejects any param absent from the registry) would silently drop working filters.

Note: Closes #119 will auto-close this issue on merge. A follow-up issue should track deferred Step 2.

Test plan

• python -m pytest tests/test_premium.py -q -m "not integration" -> 6 passed
• python -m pytest -q -m "not integration" -> 113 passed, 11 skipped, 112 deselected
• flake8 datamaxi/datamaxi/premium.py tests/test_premium.py -> clean
• black --check on both files -> unchanged

Updated test_premium_call_param_name_translation to assert snake_case wire keys (source_exchange, target_quote) and fixed the misleading comment.