fix(api): expose upsert API

[albert20260301]

Summary

• Rename the public file write API from client.upload to client.upsert.
• Rename UploadMode to UpsertMode.
• Remove the upsert content_type argument and stop sending a Content-Type header for raw upsert bodies.
• Keep the /upsert endpoint and optional mode query parameter.

Impact analysis

• Public API changes: callers now use client.upsert(...); mode enum import changes to UpsertMode.
• Tests/docs changed: tests and README use upsert naming and no content-type option.
• Cross-SDK scope: same feedback is applied to Go, JS, Ruby, Rust, and PHP.

Validation

• python3 -m compileall src tests
• Not run locally: pytest; Python exists but pip/test dependencies are not installed in this container.

[albert20260301]

@leo-altertable could you review this when you have a slot? CI is green and this is the Lakehouse /upload to /upsert endpoint update for Python.