fix(linter/unicorn/numeric-separators-style): report misgrouped BigInt literals (#27207)

### Bug

`unicorn/numeric-separators-style` checks number literals with
`formatted != raw`, but for BigInt literals it only compares lengths:
`formatted.len() != number.span.size()`. A BigInt with the same number
of separators in the wrong places has the same length as the correct
form, so it is never reported:

```js
const a = 12_34_567;   // reported, fixed to 1_234_567
const b = 12_34_567n;  // not reported (should become 1_234_567n)
const c = 0xA_BCDn;    // not reported (should become 0xAB_CDn)
```


Co-authored-by: breken-ai <312387581+breken-ai@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/oxc_linter/src/rules/unicorn/numeric_separators_style.rs

@@ -265,7 +265,7 @@ impl Rule for NumericSeparatorsStyle {
 
                 let formatted = self.format_bigint(number, raw);
 
-                if formatted.len() != number.span.size() as usize {
+                if formatted != raw {
                     ctx.diagnostic_with_fix(
                         numeric_separators_style_diagnostic(number.span),
                         |fixer| fixer.replace(number.span, formatted),
@@ -737,6 +737,8 @@ fn test() {
         ("const foo = 1_9_223n", None),
         ("const foo = 80_7n", None),
         ("const foo = 123456789_100n", None),
+        ("const foo = 12_34_567n", None),
+        ("const foo = 0xA_BCDn", None),
         ("const foo = 1_2_345_678", None),
         ("const foo = 12_3", None),
         ("const foo = 1234567890", None),
@@ -828,6 +830,8 @@ fn test() {
         ("const foo = 1_9_223n", "const foo = 19_223n", None),
         ("const foo = 80_7n", "const foo = 807n", None),
         ("const foo = 123456789_100n", "const foo = 123_456_789_100n", None),
+        ("const foo = 12_34_567n", "const foo = 1_234_567n", None),
+        ("const foo = 0xA_BCDn", "const foo = 0xAB_CDn", None),
         ("const foo = 1_2_345_678", "const foo = 12_345_678", None),
         ("const foo = 12_3", "const foo = 123", None),
         ("const foo = 1234567890", "const foo = 1_234_567_890", None),

crates/oxc_linter/src/snapshots/unicorn_numeric_separators_style.snap

@@ -107,6 +107,20 @@ source: crates/oxc_linter/src/tester.rs
    ╰────
   help: Group digits with numeric separators (_) so longer numbers are easier to read.
 
+  ⚠ unicorn(numeric-separators-style): Invalid group length in numeric value.
+   ╭─[numeric_separators_style.tsx:1:13]
+ 1 │ const foo = 12_34_567n
+   ·             ──────────
+   ╰────
+  help: Group digits with numeric separators (_) so longer numbers are easier to read.
+
+  ⚠ unicorn(numeric-separators-style): Invalid group length in numeric value.
+   ╭─[numeric_separators_style.tsx:1:13]
+ 1 │ const foo = 0xA_BCDn
+   ·             ────────
+   ╰────
+  help: Group digits with numeric separators (_) so longer numbers are easier to read.
+
   ⚠ unicorn(numeric-separators-style): Invalid group length in numeric value.
    ╭─[numeric_separators_style.tsx:1:13]
  1 │ const foo = 1_2_345_678