stanc_options values are single-quoted for make but passed verbatim to stanc, breaking compilation

[jgabry]

This bug description was generated with the help of Claude

Describe the bug

compile() builds stanc_built_options with single quotes around named option
values (R/model.R:719):

stanc_built_options <- c(stanc_built_options, paste0("--", option_name, "=", "'", stanc_options[[i]], "'"))

That quoting is correct for the STANCFLAGS += ... string handed to make, but the
same vector (stancflags_combined) is then passed straight to processx via
get_standalone_hpp() at R/model.R:728-731:

stancflags_standalone <- c("--standalone-functions", stanc_inc_paths, stancflags_combined)
self$functions$hpp_code <- get_standalone_hpp(temp_stan_file, stancflags_standalone)
private$model_methods_env_ <- new.env()
private$model_methods_env_$hpp_code_ <- get_standalone_hpp(temp_stan_file, c(stanc_inc_paths, stancflags_combined))

There is no shell in that path, so stanc receives the quotes as literal
characters and rejects the value. get_standalone_hpp() returns NULL
invisibly on failure, and the NULL blows up downstream.

This affects any named, non-logical stanc_option other than name (which is
already special-cased at R/model.R:716). Lines 728-731 run on every compile,
not only when compile_standalone = TRUE.

This is the same class of bug as #820 (fixed in #1226 for include paths), just
for option values rather than include paths.

To Reproduce

library(cmdstanr)

d <- tempdir()
f <- file.path(d, "m.stan")
writeLines(c(
  "functions { real half(real x) { return x/2; } }",
  "parameters { real y; }",
  "model { y ~ std_normal(); }"
), f)

mod <- cmdstan_model(f, stanc_options = list(canonicalize = "deprecations"))
#> Error in writeLines(private$model_methods_env_$hpp_code_, con = wsl_safe_path(private$hpp_file_,  :
#>   can only write character objects

The underlying stanc rejection, confirmed by invoking the binary directly:

stanc <- file.path(cmdstan_path(), "bin", "stanc")
processx::run(stanc, c(f, "--auto-format", "--canonicalize='deprecations'"),
              error_on_status = FALSE)$stderr
#> stanc: option --canonicalize: invalid element in list ('deprecations'): invalid ...

Passing the same option unquoted (--canonicalize=deprecations) succeeds.

Expected behavior

Compilation succeeds and stanc receives --canonicalize=deprecations. Quoting
should be applied only on the make/STANCFLAGS path, matching how include paths
are handled by the standalone_call argument of include_paths_stanc3_args().

Secondarily: a failure inside get_standalone_hpp() should surface as a real
error rather than a silent NULL that produces "can only write character
objects" several frames later.

*** Additional context
Found while reviewing #1226. Not introduced by that PR — present on master.