Keep ossutil parser compatible with release tags

[Open-Squilla]

Scope

• Inline the strict ossutil JSON parser in the mirror workflow so a mirror run can audit an immutable release tag without depending on files added after that tag.
• Accept ossutil 2.x's trailing elapsed-time line and reject other trailing output for versioning and head-object responses.

Context

The first retry used the shared parser from main, but the workflow intentionally checks out v2026.08.21.2 for source-contract auditing. That tag predates the helper file, so the run failed before any OSS upload with ModuleNotFoundError. This change keeps the workflow self-contained at the reviewed release tag.

Tests

• uv run python -m pytest tests -q — 53 passed
• python3 -m ruff check scripts tests — passed
• The helper was validated against real ossutil 2.3.0 versioning and head-object responses.

Safety

• No OSS object was uploaded or changed by the failed retry.
• No secrets are included in this PR.