Avoid mutating passed-in request data

[LeMyst]

Convert and copy incoming request payloads to prevent accidental mutation of caller-provided mappings. In mediawiki_api_call, kwargs['data'] is wrapped with dict(...) so updates (like adding format) work for non-dict mappings and don't modify the original. In mediawiki_api_call_helper, data is shallow-copied before use to avoid side effects when modifying the payload. This makes API helper functions safer when callers pass in shared or immutable mapping types.