Update fixtureId to FIXTURE_ID for consistency

[MohamadSalman11]

Description

Updated 'fixtureId' to 'FIXTURE_ID' in getFixture for consistency with constant variable naming convention

Motivation & Context

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

Related issues

[julien-deramond]

Thanks for the PR, @MohamadSalman11!
This change enhances consistency with the codebase by following the existing convention for module-level constants, particularly those used as selectors or identifiers. It aligns well with similar constants like EVENT_KEY, SELECTOR_*, etc., ensuring uniformity across the project.