rx.Var.create(x)._replace(_var_data=...) raises "TypeError: dataclasses.replace() got multiple values for keyword argument '_var_data'"

[masenf]

Describe the bug

Var._replace documents _var_data as a replaceable field, but passing it raises because _replace already supplies _var_data (merged from the existing var) to dataclasses.replace() and the caller's keyword is passed a second time.

To Reproduce

import reflex as rx
from reflex.vars import VarData

v = rx.Var.create(5)
v._replace(_var_data=VarData(imports={}))
# TypeError: dataclasses.replace() got multiple values for keyword argument '_var_data'

rx.Var.create(5, _var_data=VarData(...)) works.

Expected behavior

_replace(_var_data=...) replaces (or merges) the var data, or the signature rejects the keyword with a message that points at merge_var_data / Var.create(..., _var_data=...).

Specifics

• Reflex 0.9.12a1 (reflex-base 0.9.12a1); pre-existing, identical on 0.9.11.post1
• Python 3.11.15

Additional context

0.9.12a1 pre-release QA, vars_typing cluster, observation O5 in vars_typing/NOTES.md. Related: #3531 tightened the accepted _replace kwargs.

[linear-code]

ENG-12650