refactor(chat): add explicit return types to selector and utility fun…

[nitin-kumar-code]

Summary:
This PR adds explicit return type annotations to several selector and utility functions in react/features/chat/functions.ts.
The changes are purely type-level improvements and do not modify runtime behavior.

What was Changed:

• Added explicit return types to different functions.
• Ensured consistent and predictable return signatures across the module

Why It Is helpful:

1. Improves Type Safety
2. Makes Refactoring Safer
3. Improves Code Readability
4. Moves Codebase Toward Stricter TS Config

Behavior Impact:

✅ No runtime changes
✅ No UI changes
✅ No performance impact
✅ No breaking API changes

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.