fix(formatter_toml): handle BOM like other languages (#27775)

Use `split_bom` and restore as like other formatters.

apps/oxfmt/test/api/bom.test.ts

@@ -11,6 +11,7 @@ describe("BOM handling", () => {
     ["a.yaml", "\uFEFFkey:   value", "\uFEFFkey: value\n"],
     ["a.graphql", "\uFEFF{ a }", "\uFEFF{\n  a\n}\n"],
     ["a.json", '\uFEFF{"a":1}', '\uFEFF{ "a": 1 }\n'],
+    ["a.toml", "\uFEFFa   =   1", "\uFEFFa = 1\n"],
   ];
 
   it.each(cases)("preserves a leading BOM in %s", async (filename, source, expected) => {

crates/oxc_formatter_toml/AGENTS.md

@@ -30,7 +30,6 @@ Strong motivations, since these break rules every other language follows:
   - Other languages' IR is printed by the parent's printer, so their width counts from the embedding column (e.g. inside a list item)
   - Prettier with the plugin behaves the same as now, but consistency across our languages comes first
   - Pinned in `apps/oxfmt/test/api/markdown.test.ts`
-- No BOM split, which the other crates do
 
 Known limits of the string embedding (text with literal lines, the same as Prettier's `replaceEndOfLine()` on the plugin's string):
 

crates/oxc_formatter_toml/src/format.rs

@@ -1,6 +1,8 @@
 use oxc_allocator::ArenaVec;
 use oxc_diagnostics::OxcDiagnostic;
-use oxc_formatter_core::{EmbeddedIr, FormatSession, push_text_with_literal_lines};
+use oxc_formatter_core::{
+    EmbeddedIr, FormatSession, push_text_with_literal_lines, spec::split_bom,
+};
 use oxc_span::Span;
 
 use crate::options::TomlFormatOptions;
@@ -10,8 +12,10 @@ use crate::options::TomlFormatOptions;
 /// # Errors
 /// Returns an [`OxcDiagnostic`] when the parse produces any error.
 pub fn format(source_text: &str, options: TomlFormatOptions) -> Result<String, OxcDiagnostic> {
+    let (has_bom, source_text) = split_bom(source_text);
     let parse = parse_for_format(source_text)?;
-    Ok(oxc_toml::format(&parse, options.to_oxc_toml()))
+    let formatted = oxc_toml::format(&parse, options.to_oxc_toml());
+    Ok(if has_bom { format!("\u{feff}{formatted}") } else { formatted })
 }
 
 /// Format `source_text` into IR for embedding into another formatter's document