fix: create upload example fixtures in private directories

[apcha-oai]

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.

[openai-sdks]

OkTest Summary

✅ 237/237 SDK tests passed in 11.311s for Python SDK PR #3686.

Test results — 42 files

Test	Result	Time
tests/chat-completions-complex-body.test.ts	✅ Passed	170ms
tests/chat-completions-create.test.ts	✅ Passed	223ms
tests/chat-completions-stream.test.ts	✅ Passed	132ms
tests/files-content-binary.test.ts	✅ Passed	223ms
tests/files-create-multipart.test.ts	✅ Passed	198ms
tests/files-list-pagination.test.ts	✅ Passed	166ms
tests/initialize-config.test.ts	✅ Passed	175ms
tests/instance-isolation.test.ts	✅ Passed	206ms
tests/models-list.test.ts	✅ Passed	143ms
tests/responses-background-lifecycle.test.ts	✅ Passed	259ms
tests/responses-body-method-errors.test.ts	✅ Passed	329ms
tests/responses-cancel-timeout.test.ts	✅ Passed	190ms
tests/responses-cancel.test.ts	✅ Passed	233ms
tests/responses-compact-retries.test.ts	✅ Passed	382ms
tests/responses-compact.test.ts	✅ Passed	298ms
tests/responses-create-advanced-stream.test.ts	✅ Passed	403ms
tests/responses-create-advanced.test.ts	✅ Passed	216ms
tests/responses-create-disconnect.test.ts	✅ Passed	1.183s
tests/responses-create-errors.test.ts	✅ Passed	207ms
tests/responses-create-malformed-api-responses.test.ts	✅ Passed	267ms
tests/responses-create-retries.test.ts	✅ Passed	288ms
tests/responses-create-stream-failures.test.ts	✅ Passed	126ms
tests/responses-create-stream-timeout.test.ts	✅ Passed	237ms
tests/responses-create-stream-wire.test.ts	✅ Passed	3.793s
tests/responses-create-stream.test.ts	✅ Passed	99ms
tests/responses-create-terminal-states.test.ts	✅ Passed	226ms
tests/responses-create-timeout.test.ts	✅ Passed	203ms
tests/responses-create.test.ts	✅ Passed	240ms
tests/responses-delete.test.ts	✅ Passed	218ms
tests/responses-input-items-errors.test.ts	✅ Passed	406ms
tests/responses-input-items-list.test.ts	✅ Passed	337ms
tests/responses-input-items-options.test.ts	✅ Passed	371ms
tests/responses-input-tokens-count-timeout.test.ts	✅ Passed	228ms
tests/responses-input-tokens-count.test.ts	✅ Passed	214ms
tests/responses-malformed-inputs.test.ts	✅ Passed	2.871s
tests/responses-not-found-errors.test.ts	✅ Passed	269ms
tests/responses-parse.test.ts	✅ Passed	422ms
tests/responses-retrieve-retries.test.ts	✅ Passed	290ms
tests/responses-retrieve.test.ts	✅ Passed	208ms
tests/responses-stored-method-errors.test.ts	✅ Passed	542ms
tests/retry-behavior.test.ts	✅ Passed	3.148s
tests/sdk-error-shape.test.ts	✅ Passed	320ms

View OkTest run #32301823806

_{SDK merge (21a99596d083) · head (604d735ff685) · base (600aa8daade4) · OkTest (2b1bdfd25e98)}

[jbeckwith-oai]

Independent two-pass security review of the complete true-merge-base diff found no actionable issues. Upload examples use private temporary directories, restrictive fixture permissions and exclusive creation, reject existing/symlinked paths, clean up on failures/signals, preserve explicitly supplied caller-owned files, and exercise both upload modes with meaningful regression coverage. No SDK runtime, dependency, or API changes; substantive CI and CodeQL are green.