#7897 Add public component ref Var helper

Closes #7848

Summary

• Add rx.vars.ref() as a public helper for accessing registered component React refs.
• Reuse Reflex's canonical ref-id sanitization.
• Automatically include the required refs state import.
• Add regression coverage and documentation.

Validation

• 7 focused tests passed
• Ruff check passed
• Ruff format check passed
• Pyright passed with 0 errors
• git diff --check passed