fix(studio): fall back to TOS intranet endpoint

[evanlowe]

Summary

• probe the default Volcengine public TOS endpoint once and fall back to tos-{region}.ivolces.com only for transport-level failures
• reuse the endpoint-aware client factory across Studio storage consumers while leaving BytePlus and custom endpoints unchanged
• keep browser-facing signed URLs on the public endpoint and document the behavior

Testing

• uv run pre-commit run --show-diff-on-failure --color=always --all-files
• uv run pytest -n 16 (2929 passed, 5 skipped)