Extend http upsert keys and cursor for /upload

[fvaleye]

Summary

Extend the mock's upsert and upload contracts
Brings altertable-mock in line with the HTTP lakehouse contract: comma-separated primary_key and cursor_field on /upsert,mode=create_append on /upload.

The new contract:

• composite primary_key plus optional cursor_field on /upsert, both accepted as a single column or a comma-separated list, parsed with the backend's rule: split on commas, trim, drop empty elements, and return 400 naming the parameter when nothing is left
• mode=create_append on /upload, creating the table if missing, else appending
• uploads insert by column name, so a file narrower than an evolved table still lands and the missing columns fill with NULL
• upsert creates its missing target table like the real API