fix(studio): scope thin releases by provider

[zyn080302]

Summary

• keep the existing thin_bundles workflow input for compatibility
• apply that input only to the Volcengine publisher and keep BytePlus full-only
• add a structural YAML regression test for the provider matrix and environment binding

Context

Publish Studio Release run #826 enabled thin bundles globally. The BytePlus Release Server intentionally has thin releases disabled, so its accepted release job failed before bundle publication while the Volcengine job succeeded.

Testing

• pytest -q tests/test_studio_release_workflow.py
• ruff check tests/test_studio_release_workflow.py
• ruff format --check tests/test_studio_release_workflow.py
• pre-commit run --files .github/workflows/publish-studio-release.yaml tests/test_studio_release_workflow.py
• git diff --check

[zyn080302]

Superseded by #1079. The provider-scoped BytePlus full-only workflow fix is now included in that combined PR together with the Studio update and Harness performance fixes. The combined head has completed CI successfully.