refactor(convert_output): remove {httr} dependency and switch URL validation to base R

[Copilot]

convert_output() used {httr} only for URL existence checks, making {httr} an unnecessary runtime dependency. This PR replaces that check with base R (utils::curlGetHeaders) and removes {httr} from package imports.

• URL validation refactor (base R)

  • Replaced httr::HEAD() + httr::status_code() with utils::curlGetHeaders() in convert_output().
  • Kept existing behavior for invalid URLs by mapping header-fetch failures to 404 and emitting cli::cli_abort("Invalid URL.").

• Dependency cleanup

  • Removed httr from DESCRIPTION Imports.

• Focused coverage update

  • Added a testthat case asserting an invalid URL input triggers "Invalid URL.".

check <- tryCatch({
  headers <- utils::curlGetHeaders(file)
  attributes(headers)[["status"]]
}, error = function(e) {
  404
})
if (check == 404) cli::cli_abort(c(message = "Invalid URL."))

[Schiano-NOAA]

LGTM. Going to wait for checks to run before merging