fix(formatter): keep JSX expression child flat when fill expands its separator

Fixes #21916

When formatting a JSX child list, the printer's `Fill` correctly determined
that an expression entry (e.g. `{toCash(...).display_cents}`) fit in flat
mode even though the trailing separator needed to break. While printing
that flat entry, however, inner groups re-ran their own `fits` check using
`AllPredicate`, which walked past the entry boundary into subsequent fill
entries — frequently overshooting `printWidth` and causing the expression
group to needlessly expand into `{ \n expr \n }`.

This adds an `in_flat_fill_item` flag on the printer state that's set while
`print_fill_item` is printing a flat entry. Inner regular groups honor it
and skip re-measuring (since the fill already verified the entry fits),
while `BestFitting` still goes through its variant selection so JSX child
lists with both flat and expanded variants can still pick the right form.

Prettier conformance: no regressions (746/753 JS, 591/601 TS — unchanged).

Co-authored-by: Claude <[email protected]>

Author: jsmecham

crates/oxc_formatter/tests/fixtures/js/jsx/issue-21916.jsx

@@ -0,0 +1,13 @@
+// JSX expression child whose flat width fits on one line should not be
+// expanded just because the surrounding fill needs a separator break.
+const Component = () => {
+  return (
+    <Outer>
+      <Inner>
+        <br />
+        {format(viewModelData.the_long_property_name_for_test).display_value} (
+        {translate("labels.the_long_property_name_for_test")})
+      </Inner>
+    </Outer>
+  );
+};

crates/oxc_formatter/tests/fixtures/js/jsx/issue-21916.jsx.snap

@@ -0,0 +1,54 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+// JSX expression child whose flat width fits on one line should not be
+// expanded just because the surrounding fill needs a separator break.
+const Component = () => {
+  return (
+    <Outer>
+      <Inner>
+        <br />
+        {format(viewModelData.the_long_property_name_for_test).display_value} (
+        {translate("labels.the_long_property_name_for_test")})
+      </Inner>
+    </Outer>
+  );
+};
+
+==================== Output ====================
+------------------
+{ printWidth: 80 }
+------------------
+// JSX expression child whose flat width fits on one line should not be
+// expanded just because the surrounding fill needs a separator break.
+const Component = () => {
+  return (
+    <Outer>
+      <Inner>
+        <br />
+        {format(viewModelData.the_long_property_name_for_test).display_value} (
+        {translate("labels.the_long_property_name_for_test")})
+      </Inner>
+    </Outer>
+  );
+};
+
+-------------------
+{ printWidth: 100 }
+-------------------
+// JSX expression child whose flat width fits on one line should not be
+// expanded just because the surrounding fill needs a separator break.
+const Component = () => {
+  return (
+    <Outer>
+      <Inner>
+        <br />
+        {format(viewModelData.the_long_property_name_for_test).display_value} (
+        {translate("labels.the_long_property_name_for_test")})
+      </Inner>
+    </Outer>
+  );
+};
+
+===================== End =====================