fix: create upload example fixtures in private directories (#3686)

## Summary

- Generate each upload fixture pair in a fresh private temporary
directory, refuse existing output files, and clean up incomplete
generation.
- Print the generated directory for explicit use by the upload example;
retain disk and in-memory modes and document caller cleanup.
- Add a bounded byte-count option and focused local regression tests. No
SDK runtime, API, dependency, Python-version, or generated-code changes;
no Castiron follow-up is needed.

## Validation

- 20 focused tests passed using fake/local fixtures and a mocked upload
client.
- Shell syntax, Python compilation, repository-wide Ruff, focused
formatting, mypy, and repository-pinned Pyright passed.
- Independent read-only Codex review round 1 was clean (no actionable
P0–P2 findings).
- No live-service probes or full-size fixture generation.

This is an independent PR against public main. SDK CODEOWNER review
requested for the example file-handling change.