fix: validate client state JavaScript identifiers
HA
19h ago+123+123+012345678901234567890123456789-0-0-0123456789
ClientStateVar names are used as JavaScript bindings, so invalid identifiers or names that conflict with reserved words and generated Reflex bindings can produce broken frontend code.
Validate names when creating a ClientStateVar, and add tests covering rejected names and valid identifiers.
Fixes #6828.
Tests: 27 targeted tests passed; isolated Ruff check passed.