Split file import API by format

The V2 import API now has a dedicated endpoint for each kind of file, so structured datalogs and tabular spreadsheets each get a request shape that fits them.

• Added POST /v2/imports/structured to import 1 to 100 OpenHTF, WATS, ATML, NI TestStand, STDF, or ATDF files in one call, each parsed independently so one bad file never fails the rest.
• Added POST /v2/imports/tabular to import a CSV or Excel file with an inline column mapping or a saved template.
• Updated every SDK client (Python, C#, Rust, C++, MATLAB) with typed methods for both endpoints.