feat(studio): provision identity resources on deploy

[evanlowe]

Summary

• create or reuse a Studio Identity user pool and web Client when IDs are omitted
• provision resources in the selected deployment region for Volcengine and BytePlus
• print the resolved Identity region, user pool ID/domain, and Client ID without exposing the Client Secret
• preserve existing-resource behavior and document the optional flags

Verification

• Studio deploy target tests: 40 passed
• pre-commit: ruff check, ruff format, and hardcoded-secret scan passed
• CI: all required checks passed on Python 3.10 and 3.12
• real AK/SK Identity lifecycle: create, read back, reuse, secret-output check, and cleanup passed
  • Volcengine cn-beijing
  • Volcengine cn-shanghai
  • BytePlus ap-southeast-1
• real full Studio deploy with current branch: Volcengine cn-beijing passed, including callback registration, final ID output, HTTP 200, and cleanup
• real BytePlus Identity creation and reuse passed in ap-southeast-1
• BytePlus full source deployment is not marked passed: the 39.31 MB source bundle exceeded the current 300-second function upload timeout; all test resources were cleaned