Avoid Float out of range warning for clearly underflowing numbers

When parsing JSON floats with extremely negative exponents (like
123.456e-789 or 123e-10000000), the parser would fall back to
rb_cstr_to_dbl which internally calls strtod. When strtod returns
ERANGE due to underflow to 0.0, Ruby emits a "Float out of range"
warning, causing noise in the test output.

Fix: when mantissa_digits + exponent < -324, the effective value is
less than 10^(-324) < DBL_TRUE_MIN/2, so it must round to 0.0 in
IEEE 754 round-to-nearest. Return 0.0 directly without going through
rb_cstr_to_dbl, avoiding the spurious warning.

This fixes warnings introduced by JSONMinefieldParserTest tests
(test_i_number_double_huge_neg_exp and test_i_number_real_underflow)
added in commit 6507a836c5.

Author: Copilot

ext/json/ext/parser/parser.c

@@ -1133,6 +1133,13 @@ static inline VALUE json_decode_float(JSON_ParserConfig *config, uint64_t mantis
     }
 
     if (RB_UNLIKELY(mantissa_digits > 18 || mantissa_digits + exponent < -307)) {
+        // If the value is so small that it definitely underflows to 0.0, return early
+        // to avoid triggering a "Float out of range" warning from rb_cstr_to_dbl.
+        // When mantissa_digits + exponent < -324, value < 10^(-324) < DBL_TRUE_MIN/2,
+        // so it rounds to 0 in IEEE 754 round-to-nearest.
+        if (RB_UNLIKELY(mantissa_digits + exponent < -324)) {
+            return rb_float_new(negative ? -0.0 : 0.0);
+        }
         return json_decode_large_float(start, end - start);
     }