feat(core): add strict validation for with_vars conflicts(Fixes #688)

[Tsuyoposon]

Brief

Fixes #688
This PR introduces a strict validation (fail-fast mechanism) when merging with_vars to prevent silent overwrites.

Previously, if the same with_vars key was defined in multiple places (e.g., across multiple common files passed to ScenarioManager, or between global and step scopes), the framework silently overwrote the value. This PR updates ScenarioModel and _BaseWithVars to explicitly raise an InvalidFormat exception whenever a with_vars conflict is detected.

Note: The shallow-merge behavior for step arguments remains unchanged.

Points to Check

• Raised InvalidFormat in _BaseWithVars._merge_static_vars when there is a scope conflict between global and step levels.
• Raised InvalidFormat in ScenarioModel.merge when there is a global with_vars conflict across multiple files.
• Raised InvalidFormat in ScenarioModel._merge_step when there is a step-level with_vars conflict across multiple files.

Test

Confirmed

Updated test_model.py to cover both normal scenarios (successful merges without overlaps) and abnormal scenarios (triggering InvalidFormat on with_vars conflicts). All tests and lint.sh checks (including Black, isort, and flake8) passed successfully.

Review Limit

• Around 06/30

[skxeve]

Thanks for your PR!
LGTM!