rust(fix): use job ID from upload response for deterministic lookup

[wei-qlu]

Summary

• Replaces the "latest job for user" lookup with deterministic retrieval by job_id
• job_id is now read from the upload response body and threaded through wait_for_job_completion
• Consolidates the inline upload code in csv.rs and parquet/flat_dataset.rs to use the existing upload_gzipped_file helper
• Removes user.rs helper (only used by the old user-scoped "latest job" lookup)

Validation

• Manual: sift-cli import csv --wait ... completes and reports the correct job status
• Manual: same for import parquet, import hdf5, import tdms
• Manual: temporarily printed job_id and confirmed the printed id matches the job created by the upload in the Sift jobs UI (not a stale or unrelated job)