Make LongBridge API probe credential loading flexible

[Pigbibi]

Summary

• let the manual LongBridge API probe use GitHub Environment secrets when present
• keep Secret Manager as the normal source, with a runtime service account impersonation fallback for deployments where the OIDC deploy service account cannot read secret versions directly
• document the alternate credential source and expand workflow structure coverage

Validation

• /home/ubuntu/Projects/LongBridgePlatform/.venv/bin/python -m unittest tests.test_longbridge_api_probe_workflow -v
• bash tests/test_longbridge_api_probe_workflow.sh
• /home/ubuntu/Projects/LongBridgePlatform/.venv/bin/python -m ruff check tests/test_longbridge_api_probe_workflow.py
• git diff --check

Context

The first manual HK probe run reached Google Cloud auth but failed before broker API execution because longbridge-platform-deploy@longbridgequant does not have secretmanager.versions.access on the LongPort credential secrets.