fix: add trailing comma after wrapped arrow expression body in call (#696)

[todor-a]

Fixes #696.

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Transparency note: This PR was prepared with AI assistance (Claude).

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Problem

With trailingCommas: always:

const test = ["a","b","c"].map(value =>
  value == "a quite long paragraph with many words")

…formats to (no comma after the arrow body):

const test = ["a", "b", "c",].map(value =>
  value == "a quite long paragraph with many words"
)

Issue reports the expected output should be:

const test = ["a", "b", "c",].map(value =>
  value == "a quite long paragraph with many words",
)

Root cause

gen_parameters_or_arguments (src/generation/generate.rs) has a special branch for calls with a single argument that is an arrow function with an expression body:

if !force_use_new_lines && nodes.len() == 1 && is_arrow_function_with_expr_body(nodes[0]) {
    …
    items.push_condition(if_true_or(
      "isDifferentLineAndStartLineIndentation",
      …,
      Signal::NewLine.into(),                    // <-- true branch: just newline
      if space_around { … } else { … },
    ));
}

When the arrow body forces the call onto multiple lines, the true branch emits a bare Signal::NewLine before the closing paren — no comma, regardless of trailingCommas. The fallthrough gen_separated_values branch (which honors trailingCommas via separator: trailing_commas.into()) is bypassed.

Fix

Inject , into the true branch when trailingCommas resolves to always:

let trailing_break_items = {
  let mut break_items = PrintItems::new();
  if matches!(trailing_commas, TrailingCommas::Always) {
    break_items.push_sc(sc!(","));
  }
  break_items.push_signal(Signal::NewLine);
  break_items
};
items.push_condition(if_true_or(
  "isDifferentLineAndStartLineIndentation",
  …,
  trailing_break_items,
  …,
));

Scope is limited to always. onlyMultiLine keeps existing behavior — extending it would update several existing specs (issue105, issue165, JsxElement_MultiLineParens_Never, Arguments_SpaceAround_True) and might surprise users running the default config. Could be revisited as a follow-up.

Tests

Corner-case probe also based on prettier's tests/format/js/trailing-comma (function-calls.js, dynamic-import.js, jsx.js).