OAuth clients with custom `auth` functions can't be printed

[atheriel]

The oauth_client() function accepts custom auth functions, but using them results in an object that can't be printed:

library(httr2)

oauth_client(
  "example",
  "localhost:8080/token",
  secret = "secret",
  auth = "header"
)
#> <httr2_oauth_client>
#> name: 36788c18332cb6566bf0534e8048fce6
#> id: example
#> secret: <REDACTED>
#> token_url: localhost:8080/token
#> auth: oauth_client_req_auth_header

oauth_client(
  "example",
  "localhost:8080/token",
  auth = function(req, client) {
    req_headers(req, `x-auth` = "secret")
  }
)
#> <httr2_oauth_client>
#> Error in "cli::cli_dl(redacted)": ! Could not evaluate cli `{}` expression: ` req_headers(re…`.
#> Caused by error in `eval(expr, envir = envir)`:
#> ! object 'req' not found

^{Created on 2025-01-21 with reprex v2.1.1}

[hadley]

Ooops, I bet it's because I didn't use the correct technique to print text that contains {}.