fix: replace deprecated String.Prototype.substr with slice

[Sam-61s]

What kind of change does this PR introduce?

• Bug fix / Refactoring

Issue Number:

• Closes 🐛 Bug: Replace deprecated String.substr usages with slice #2006

Screenshots/videos:

• Not applicable (no UI changes)

If relevant, did you update the documentation?

• Not required

Summary
This PR replaces usages of the deprecated String.prototype.substr API with standard and well-supported alternatives (slice or substring) in the affected areas.

The deprecated usage was causing ESLint/editor warnings. The changes preserve existing behavior and logic while aligning with modern JavaScript best practices.

Note on failing test
There is a pre-existing test failure in JsonEditor.cy.tsx for the router navigation test
(should navigate correctly to each type-keyword reference page when clicked on type-keyword).

This failure exists on the base branch and is unrelated to this change.
It was verified by stashing these changes and running the tests on the base branch, where the failure still occurs.

Does this PR introduce a breaking change?

• No

Checklist

• Read, understood, and followed the contributing guidelines

[github-actions]

built with Refined Cloudflare Pages Action

⚡ Cloudflare Pages Deployment

Name	Status	Preview	Last Commit
website	✅ Ready (View Log)	Visit Preview	70588e6

[Sam-61s]

@Utkarsh-123github
The failing JsonEditor.cy.tsx router navigation test is pre-existing on the base branch and unrelated to this change.
This PR replaces deprecated String.prototype.substr usage while preserving existing behavior.
Details are noted in the PR description.