feature: adding smarter unit test execution

[cmgoffena13]

Description

During plan, run unit tests only for models in the plan change set (added, modified, restated).
No-change plans skip tests by default.
--all-tests runs the full suite (previous default).

Addresses #5965

Test Plan

• Plan with model changes runs only those models’ unit tests
• Plan with no changes skips unit tests
• --all-tests runs the full suite (including no-change plans)
• --skip-tests still skips; incompatible with --all-tests
• Docs cover plan-scoped default and --all-tests

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