Handle stan_threads cpp option consistent with cmdstan

* mod$cpp_options() now shows options as false if unset
    (same as the `info` cli command). Previously, this
    was inconsistent: sometimes showing the user-provided arg
    and sometimes showing the output of `info`
* tests now match expected behavior of cmdstan

R/model.R

@@ -738,7 +738,7 @@ compile <- function(quiet = TRUE,
   } # End - if(!dry_run)
 
   private$exe_file_ <- exe
-  private$cpp_options_ <- cpp_options
+  private$cpp_options_ <- model_compile_info(exe)
   private$precompile_cpp_options_ <- NULL
   private$precompile_stanc_options_ <- NULL
   private$precompile_include_paths_ <- NULL

tests/testthat/test-threads.R

@@ -158,20 +158,21 @@ test_that("threading works with generate_quantities()", {
   expect_equal(f_gq$metadata()$threads_per_chain, 4)
 })
 
-test_that("correct output when stan_threads not TRUE", {
-  mod <- cmdstan_model(stan_program, cpp_options = list(stan_threads = FALSE), force_recompile = TRUE)
+test_that("correct output when stan_threads unset", {
+  mod <- cmdstan_model(stan_program, cpp_options = list(stan_threads = NULL), force_recompile = TRUE)
   expect_output(
     mod$sample(data = data_file_json),
     "Running MCMC with 4 sequential chains",
     fixed = TRUE
   )
   mod <- cmdstan_model(stan_program, cpp_options = list(stan_threads = "dummy string"), force_recompile = TRUE)
-  expect_output(
+  expect_error(
     mod$sample(data = data_file_json),
-    "Running MCMC with 4 sequential chains",
+    "The model was compiled with 'cpp_options = list(stan_threads = TRUE)' but 'threads_per_chain' was not set!",
     fixed = TRUE
   )
-  mod <- cmdstan_model(stan_program, cpp_options = list(stan_threads = FALSE), force_recompile = TRUE)
+
+  mod <- cmdstan_model(stan_program, cpp_options = list(stan_threads = NULL), force_recompile = TRUE)
   expect_warning(
     mod$sample(data = data_file_json, threads_per_chain = 4),
     "'threads_per_chain' is set but the model was not compiled with 'cpp_options = list(stan_threads = TRUE)' so 'threads_per_chain' will have no effect!",