Improve error messages for missing required input arguments

[samuelbharti]

Fixes #1423.

Summary

Input constructors (textInput(), sliderInput(), selectInput(), actionButton(), etc.) currently surface cryptic downstream R errors like argument "label" is missing, with no default when a required argument is omitted, without naming the function or the argument that was actually missing.

This PR adds a small internal check_required() helper in R/input-utils.R that uses eval(call("missing", ...), envir = caller_env()) to detect unsupplied promises without forcing them, then raises a classed error via cli::cli_abort() that names both the calling function and the missing argument(s). The helper is wired into the top of every *Input() constructor plus actionButton() / actionLink() for arguments without defaults (inputId, label, plus per-function specifics such as choices, min/max/value, data).

The error carries class shiny_missing_arg_error so downstream code can catch it specifically. Explicit NULL and NA are treated as "supplied" so documented opt-outs like label = NULL continue to work. No public API changes; only the error wording changes for the previously-broken cases.

Scope

In scope: every standalone input constructor and actionButton() / actionLink() — 15 functions in total.

Out of scope (intentionally):

• Argument type validation (e.g., rejecting inputId = c("a", "b")). The issue is about clearer missing-arg messages, not a general validator.
• Positional-shift heuristics. sliderInput("forgot my id!", 1, 10, 5) will still report value as missing because we can't infer the user's intent.
• The update*Input() family. Same problem exists there but the surface area is larger; tracked as a follow-up.

Follow-ups

• Apply the same check_required() treatment to updateTextInput(), updateSliderInput(), updateSelectInput(), etc.
• Consider exporting check_required() (or replacing with rlang::check_required()) if other parts of shiny adopt the pattern.

Verification

Before:

sliderInput("Whoops forgot my id!", 1, 10, 5)
#> Error in force(default) : argument "value" is missing, with no default
textInput("text")
#> Error in label %AND% tags$label(label, `for` = inputId) :
#>   argument "label" is missing, with no default

After:

sliderInput("Whoops forgot my id!", 1, 10, 5)
#> Error in `sliderInput()`:
#> ! `sliderInput()` is missing required argument: `value`.
textInput("text")
#> Error in `textInput()`:
#> ! `textInput()` is missing required argument: `label`.
textInput()
#> Error in `textInput()`:
#> ! `textInput()` is missing required arguments: `inputId` and `label`.

Test plan

• tests/testthat/test-input-required-args.R adds 7 test_that() blocks (48 expectations) covering: the helper's own behavior, the new error class, behavior under do.call(), regex-based regression assertions, explicit-NULL opt-out, happy-path shiny.tag returns for all 15 patched constructors, and full-message snapshots.
• devtools::test(filter = "input"): 231 PASS, 0 FAIL.
• Full devtools::test() suite earlier: 2018 PASS, 0 FAIL (12 environmental skips, unrelated).