Fixes infinite loop in tokenizer on trailing stray '{'

The stray-{ else branch in tokenize_fast had a nested while loop that could
exit via its condition (when next_open >= len) without advancing pos.
The outer while pos < len loop would then find the same { again forever.

Reproduced by: Liquid::Template.parse('a{') -- hangs indefinitely.

Replaces the nested scan loop with two String#byteindex calls to find the
next '{%' and '{{' directly, then takes the minimum. Always O(n), eliminates
the nested loop entirely, and impossible to leave pos stranded.

Adds regression tests covering the three inputs that previously hung
plus adjacent stray-brace cases.

lib/liquid/tokenizer.rb

@@ -146,24 +146,18 @@ module Liquid
             pos = idx + 2
           end
         else
-          # { followed by something else — it's text
-          # Keep scanning from after this {
-          # Find next { that could be {%  or {{
-          next_open = idx + 1
-          while next_open < len
-            ni = src.byteindex('{', next_open)
-            unless ni
-              @tokens << src.byteslice(pos, len - pos)
-              pos = len
-              break
-            end
-            nb = ni + 1 < len ? src.getbyte(ni + 1) : nil
-            if nb == PERCENTAGE || nb == OPEN_CURLEY
-              @tokens << src.byteslice(pos, ni - pos)
-              pos = ni
-              break
-            end
-            next_open = ni + 1
+          # Lone '{' — not the start of a tag or variable.
+          # Find the next '{{' or '{%' to know where this text token ends.
+          # Using two byteindex calls avoids a nested loop and is always O(n).
+          tag_start = src.byteindex('{%', idx + 1)
+          var_start = src.byteindex('{{', idx + 1)
+          next_token = [tag_start, var_start].compact.min
+          if next_token
+            @tokens << src.byteslice(pos, next_token - pos)
+            pos = next_token
+          else
+            @tokens << src.byteslice(pos, len - pos)
+            pos = len
           end
         end
       end

test/unit/tokenizer_unit_test.rb

@@ -48,6 +48,33 @@ class TokenizerTest < Minitest::Test
     assert_equal(["{%%}", "}"], tokenize('{%%}}'))
   end
 
+  # Regression: lone '{' at or near end of string previously caused an infinite
+  # loop. The stray-{ else branch left `pos` unchanged when no further '{{' or
+  # '{%' existed, so the outer loop found the same '{' on every iteration.
+  def test_lone_brace_does_not_loop
+    assert_equal(["{"],              tokenize('{'))
+    assert_equal(["a{"],             tokenize('a{'))
+    assert_equal(["hello { world {"], tokenize('hello { world {'))
+    assert_equal(["{ world"],         tokenize('{ world'))
+    assert_equal(["x{y"],             tokenize('x{y'))
+    assert_equal(["{b{c"],            tokenize('{b{c'))
+  end
+
+  def test_lone_brace_before_real_token
+    assert_equal(
+      ["a { b ", "{% if x %}", "yes", "{% endif %}", " c"],
+      tokenize('a { b {% if x %}yes{% endif %} c'),
+    )
+    assert_equal(
+      ["x { ", "{{ var }}", " y"],
+      tokenize('x { {{ var }} y'),
+    )
+    assert_equal(
+      ["{ ", "{{ var }}"],
+      tokenize('{ {{ var }}'),
+    )
+  end
+
   private
 
   def new_tokenizer(source, parse_context: Liquid::ParseContext.new, start_line_number: nil)