Fix OSS-Fuzz #508087118: avoid stack overflow in str2tag

str2tag_core is recursive (one frame per character), so a long runtime
input such as a fuzzer-supplied Content-Type would overflow the stack.
Rewrite the runtime entry point str2tag() iteratively while keeping the
recursive constexpr str2tag_core for compile-time UDL evaluation. The
hash output is unchanged for all inputs.

test/test.cc

@@ -767,6 +767,13 @@ TEST(ParseAcceptHeaderTest, InvalidCases) {
   EXPECT_EQ(result[0], "text/*");
 }
 
+// Regression test for OSS-Fuzz #508087118: a long Content-Type ran str2tag
+// recursively (one stack frame per character) and overflowed the stack.
+TEST(Str2tagTest, LongInputDoesNotOverflowStack) {
+  std::string long_content_type(60000, 'x');
+  EXPECT_NO_THROW(detail::can_compress_content_type(long_content_type));
+}
+
 TEST(ParseAcceptHeaderTest, ContentTypesPopulatedAndInvalidHeaderHandling) {
   Server svr;