Follow-ups from #7215: dead statement in test_reserved_mixin_var, typed _reflex_state_root default, document the type(rx.State) is BaseStateMeta contract

[masenf]

Three follow-ups recorded in the #7215 review (reserved-name validation moved into BaseStateMeta.__new__), all out of scope for that PR:

1. tests/units/test_state.py::test_reserved_mixin_var[state_mixin=True] has a dead statement. For that parametrization the reserved name is rejected at the mixin's own declaration, so the second type("MixedState", …) statement never runs. Pre-existing since Validate reserved state names before registration #7136 (the test was moved verbatim). Restructure it so each parametrization asserts where the rejection happens.
2. Replace the getattr(base, "_reflex_state_root", None) lookup over the bases in BaseStateMeta.__new__ (packages/reflex-base/src/reflex_base/vars/base.py) with a typed metaclass default (_reflex_state_root: BaseStateMeta | None = None) and limit the lookup to the EvenMoreBasicBaseState bases. Removes the string literal and the getattr.
3. Document the contract in the BaseStateMeta docstring: type(rx.State) is BaseStateMeta is supported (downstream metaclasses derive from it; reflex-enterprise's OIDCCookieMeta does), state_root= and mixin= are class keywords the metaclass understands, and _reflex_state_root is a reserved name. Its absence is what let 0.9.12a1 ship the sibling metaclass that caused 0.9.12a1: metaclass conflict for State subclasses using a BaseStateMeta-derived metaclass (breaks reflex-enterprise 0.9.5 auth/MCP/REST apps at startup) #7211.

Source: fixes/f001/REPORT.md, "PR review follow-ups", and the #7215 description.

[linear-code]

ENG-12658