read csv with variable names that contain parentheses

[yizhang-yiz]

Describe the bug
I'm trying to piggyback cmdstanr to read samples generated elsewhere. as_cmdstan_fit complains variable names such as "OMEGA(1,1)", whiledata.table::fread ingests fine. I think it's caused by the internal fread_cmd's grepping unpacking variable names.

To Reproduce
For a csv with content (dummy metadata)

# model = norm_model
# start_datetime = 2025-11-19 22:23:43 UTC
# method = sample (Default)
#   sample
#   ....
THETA4,"SIGMA(1,1)"
2.00000E+00,2.00000E+00
2.00000E+00,2.00000E+00

as_cmdstan_fit("foo.csv", check_diagnostics=FALSE) errors

Error in grep(pattern, var_names) : 
  invalid regular expression '^"SIGMA(1\[', reason 'Missing ')''
In addition: Warning message:
In grep(pattern, var_names) : TRE pattern compilation error 'Missing ')''

Expected behavior
No error, as by fread:

fread(cmd="grep -v '^#' --color=never foo.csv")
   THETA4 SIGMA(1,1)
    <num>      <num>
1:      2          2
2:      2          2

Operating system
MacOS 15.7 (24G222) 11881.140.96
Ubuntu 24.04

CmdStanR version number
cmdstanr_0.9.0 & master 7c7b5d2