fix(postgres): randomize test username and update grant tests

[sakshipatil-hue]

Description

• Randomizes PostgreSQL integration test usernames using a 6-character UUID hex suffix to prevent CI role collision issues.
• Updates test_grants_plan_target_layer_physical_only and test_grants_plan_target_layer_virtual_only to utilize dynamic usernames via f-strings rather than hardcoded strings.

Test Plan

• Verified code changes match existing dynamic test patterns in test_integration_postgres.py.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO

[cmgoffena13]

@sakshipatil-hue -- formatting is failing. I pulled the fork and confirmed make style would fix it. Make sure that you have properly setup your repo by running make install-dev after creating a virtual environment as well, pre-commit should have caught that. Let me know once you've completed the checklist.

[sakshipatil-hue]

Hi @cmgoffena13, thanks for the feedback! I've gone ahead and formatted the target file using Ruff. The branch is updated and ready for your review whenever you have a moment.