test_broken_bignum: avoid fork and subprocess for robustness

The raw fork + Process.waitpid2 pattern causes a SEGV on i686-linux in
ruby/ruby CI. The crash occurs in waitpid_blocking_no_SIGCHLD going
through the 32-bit vdso, which is a Ruby VM / kernel interaction issue
unrelated to the json gem itself.

Replace the fork-based approach with an in-process test that temporarily
patches Integer#to_s and restores it via ensure. This avoids the fork
SEGV on i686, works under valgrind (assert_separately doesn't), and
is guarded with `if defined?(JSON::Ext::Generator)` to skip on
JRuby/TruffleRuby where the C extension isn't available.

Author: k0kubun

test/json/json_generator_test.rb

@@ -475,25 +475,20 @@ def foo.to_h
     assert_equal '2', state.indent
   end
 
-  def test_broken_bignum # [ruby-core:38867]
-    pid = fork do
-      x = 1 << 64
-      x.class.class_eval do
+  if defined?(JSON::Ext::Generator)
+    def test_broken_bignum # [ruby-core:38867]
+      bignum = 1 << 64
+      original_to_s = bignum.class.instance_method(:to_s)
+      bignum.class.class_eval do
         def to_s
         end
       end
-      begin
-        JSON::Ext::Generator::State.new.generate(x)
-        exit 1
-      rescue TypeError
-        exit 0
+      assert_raise(TypeError) do
+        JSON::Ext::Generator::State.new.generate(bignum)
       end
+    ensure
+      bignum.class.define_method(:to_s, original_to_s) if original_to_s
     end
-    _, status = Process.waitpid2(pid)
-    assert status.success?
-  rescue NotImplementedError
-    # forking to avoid modifying core class of a parent process and
-    # introducing race conditions of tests are run in parallel
   end
 
   def test_hash_likeness_set_symbol