fix: Implement Tableau-compliant multipart publish for workbooks and data sources

[teesofttech]

Problem

Publish methods currently use MultipartFormDataContent with simple fields. Tableau publish endpoints require a multipart/mixed request with a request_payload part plus the workbook or datasource file part. Publish options such as overwrite/asJob belong in the endpoint query where applicable.

Scope

• Fix WorkbookService.PublishAsync request format.
• Fix DataSourceService.PublishAsync request format.
• Support twb/twbx and datasource file-type handling where required.
• Add validation for file path, extension, project id, and name.
• Decide whether large-file chunked upload is in scope now or a follow-up.

Tests

• Unit tests inspect outgoing content type, payload part name, file part name, and query parameters.
• Unit tests for validation failures.
• Integration test path for publish can be opt-in because it mutates Tableau content.

Done When

Publish requests match Tableau REST API requirements and are covered by focused unit tests.