fix: lazy load Resource Manager client in create flow

[he-yufeng]

Summary

• move the Resource Manager import out of module import time
• keep list_gcp_projects() returning an empty list when the optional Resource Manager dependency is unavailable
• add coverage for the missing-dependency path

Fixes #5766

To verify

• .\.venv\Scripts\python.exe -m pytest tests/unittests/cli/utils/test_gcp_utils.py -q -p no:cacheprovider
• .\.venv\Scripts\python.exe -m py_compile src\google\adk\cli\utils\gcp_utils.py tests\unittests\cli\utils\test_gcp_utils.py
• uv run --extra dev mypy --config-file pyproject.toml src\google\adk\cli\utils\gcp_utils.py
• uv run --extra dev pyink --check src\google\adk\cli\utils\gcp_utils.py tests\unittests\cli\utils\test_gcp_utils.py
• git diff --check

Notes

• uv run --extra test ... is currently blocked on Windows by yarl==1.24.1, which has no win_amd64 wheel or source distribution.

[sasha-gitg]

This is fixed in main