fix: replace indexOf existence checks with includes (SonarCloud maintainability)

[mnashawati]

Description

Please include a summary of the change:

• Replace .indexOf() !== -1 / >= 0 / === -1 patterns with .includes() across 6 source files. This addresses ~9 SonarCloud code smell issues (es6 readability).

• .includes() is functionally identical for existence checks and is more readable.

Fixes - Jira Link #
https://eaflood.atlassian.net/browse/FI0-11035

Type of change

Please tick the relevant option for your PR request. Follow Semantic Version: vMAJOR.MINOR.PATCH

• Bug fix - PATCH (non-breaking change which fixes an issue)
• New feature - MINOR (non-breaking change which adds functionality)
• Breaking change - MAJOR (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update, or other non-code files.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
90.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud