fix(formatter): Trim trailing whitespace before breaking line (#19740)

This doesn't seem to be an issue with native js/ts format paths, but it appears to cause problems when converting Prettier Docs, such as with gql-in-js.

Without this change,

```gql
type Mutation {
    create__TYPE_NAME__(input: Create__TYPE_NAME__Input!): __TYPE_NAME__!
      @skipAuth
```

will be formatted:

```gql
type Mutation {
    create__TYPE_NAME__(input: Create__TYPE_NAME__Input!): __TYPE_NAME__!
      @skipAuth
```

See trailing ` ` after `__TYPE_NAME__!`.

apps/oxfmt/test/api/xxx-in-js-conformance.test.ts

@@ -12,33 +12,21 @@ describe("Prettier conformance for graphql-in-js", () => {
     "format.test.js",
     "comment-tag.js", // /* GraphQL */
   ]);
-  // TODO: Fix in next PR
-  //   graphqlFixtures.push(
-  //     {
-  //       name: "inline/gql-trailing-space.js",
-  //       content: `export const schema = gql\`
-  //   type Mutation {
-  //     create__TYPE_NAME__(input: Create__TYPE_NAME__Input!): __TYPE_NAME__!
-  //       @skipAuth
-  //     update__TYPE_NAME__(
-  //       id: Int!
-  //       input: Update__TYPE_NAME__Input!
-  //     ): __TYPE_NAME__! @skipAuth
-  //     delete__TYPE_NAME__(id: Int!): __TYPE_NAME__! @skipAuth
-  //   }
-  // \`
-  // `,
-  //     },
-  //   );
+
   graphqlFixtures.push({
-    name: "inline/gql-dummy.js",
+    name: "inline/gql-trailing-space.js",
     content: `export const schema = gql\`
-  type Relation {
-    id: Int!
-    name: String!
+  type Mutation {
+    create__TYPE_NAME__(input: Create__TYPE_NAME__Input!): __TYPE_NAME__!
+      @skipAuth
+    update__TYPE_NAME__(
+      id: Int!
+      input: Update__TYPE_NAME__Input!
+    ): __TYPE_NAME__! @skipAuth
+    delete__TYPE_NAME__(id: Int!): __TYPE_NAME__! @skipAuth
   }
-  \`
-  `,
+\`
+`,
   });
 
   describe.concurrent.each(graphqlFixtures)("$name", ({ name, content }) => {

crates/oxc_data_structures/src/code_buffer.rs

@@ -538,6 +538,17 @@ impl CodeBuffer {
         unsafe { self.buf.set_len(len + bytes) };
     }
 
+    /// Remove trailing whitespace (spaces and tabs) from the buffer.
+    ///
+    /// This trims trailing whitespace before line breaks, matching Prettier's `trimEnd(out)` behavior.
+    /// <https://github.com/prettier/prettier/blob/90983f40dce5e20beea4e5618b5e0426a6a7f4f0/src/document/printer/printer.js#L535>
+    #[inline]
+    pub fn trim_trailing_ascii_whitespace(&mut self) {
+        while self.buf.last().is_some_and(|&b| b == b' ' || b == b'\t') {
+            self.buf.pop();
+        }
+    }
+
     /// Get contents of buffer as a byte slice.
     ///
     /// # Example

crates/oxc_formatter/src/formatter/printer/mod.rs

@@ -127,6 +127,7 @@ impl<'a> Printer<'a> {
 
                 // Only print a newline if the current line isn't already empty
                 if self.state.line_width > 0 {
+                    self.state.buffer.trim_trailing_ascii_whitespace();
                     self.print_char('\n');
                     self.state.has_empty_line = false;
                 }