Return a single diagnostic from `lex_token_trees`.

It currently returns a `Vec` but in practice it always has one
diagnostic in it.

LLM disclosure: Claude Code identified this when I asked it to review
`tokentrees.rs`. I made the change by hand and tested it myself.

compiler/rustc_parse/src/lexer/mod.rs

diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
index 5766d25..173a92f 100644
--- a/compiler/rustc_parse/src/lexer/mod.rs
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -115,7 +115,7 @@ pub(crate) fn lex_token_trees<'psess, 'src>(
         Err(errs) => {
             // We emit delimiter mismatch errors first, then emit the unclosing delimiter mismatch
             // because the delimiter mismatch is more likely to be the root cause of error
-            unmatched_closing_delims.extend(errs);
+            unmatched_closing_delims.push(errs);
             Err(unmatched_closing_delims)
         }
     }

compiler/rustc_parse/src/lexer/tokentrees.rs

diff --git a/compiler/rustc_parse/src/lexer/tokentrees.rs b/compiler/rustc_parse/src/lexer/tokentrees.rs
index bce2981..757cd75 100644
--- a/compiler/rustc_parse/src/lexer/tokentrees.rs
+++ b/compiler/rustc_parse/src/lexer/tokentrees.rs
@@ -14,7 +14,7 @@ impl<'psess, 'src> Lexer<'psess, 'src> {
     pub(super) fn lex_token_trees(
         &mut self,
         is_delimited: bool,
-    ) -> Result<(Spacing, TokenStream), Vec<Diag<'psess>>> {
+    ) -> Result<(Spacing, TokenStream), Diag<'psess>> {
         // Move past the opening delimiter.
         let open_spacing = self.bump_minimal();
 
@@ -35,11 +35,11 @@ pub(super) fn lex_token_trees(
                 return if is_delimited {
                     Ok((open_spacing, TokenStream::new(buf)))
                 } else {
-                    Err(vec![self.close_delim_err(delim)])
+                    Err(self.close_delim_err(delim))
                 };
             } else if self.token.kind == token::Eof {
                 return if is_delimited {
-                    Err(vec![self.eof_err()])
+                    Err(self.eof_err())
                 } else {
                     Ok((open_spacing, TokenStream::new(buf)))
                 };
@@ -54,7 +54,7 @@ pub(super) fn lex_token_trees(
     fn lex_token_tree_open_delim(
         &mut self,
         open_delim: Delimiter,
-    ) -> Result<TokenTree, Vec<Diag<'psess>>> {
+    ) -> Result<TokenTree, Diag<'psess>> {
         // The span for beginning of the delimited section.
         let pre_span = self.token.span;