Allow character grouping columns in factor smooths

[Copilot]

mgcv factor smooths (bs = "fs") require grouping variables to be factors, but character columns caused smooth construction errors.

• Internal coercion: Convert character variables used by bs = "fs" smooth terms to factors before calling mgcv::smoothCon().
• Regression coverage: Add a unit test for character grouping input.

data <- data.frame(
  x = seq(0, 6, length.out = 24),
  tissue = rep(c("blood", "lymph", "tumor"), length.out = 24)
)

parse_formula_smooths(~ s(x, tissue, bs = "fs", k = 5), data)

• Fixes smooth grouping should be a factor now, can we open it to be a character without causing error? #292