gh-NNNN: Add regression test for long-mantissa halfway-plus-epsilon

Covers the failure shape of a shortest-then-round strtod that
truncates its mantissa buffer without carrying a "non-zero tail was
dropped" flag through to the tie-break: the truncated form looks
exactly like a halfway case and rounds-to-even the wrong way.

The test constructs the midpoint between 1.0 and the next double
(1.0 + 2**-52), pads with 800 zeros, and appends a '1'. The value is
strictly above the halfway by 10**-857, so any bignum-capable strtod
must round UP — fast_float's Eisel-Lemire + digit-comparison fallback
handles this correctly, making the test pass as-is.

The test is placed alongside test_strtod.StrtodTests.test_particular
so it's part of the suite that already runs against every strtod
patch.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: eendebakpt

Lib/test/test_strtod.py

@@ -429,5 +429,24 @@ def test_particular(self):
         for s in test_strings:
             self.check_strtod(s)
 
+    def test_halfway_plus_epsilon_long_mantissa(self):
+        """Regression test for long-mantissa halfway-plus-epsilon cases.
+
+        The construction is "(exact midpoint between 1.0 and the next
+        double)" + 800 zeros + "1". The value sits strictly *above* the
+        halfway by 10**-857, so bignum-backed strtod must round UP to
+        1.0 + 2**-52 rather than ties-to-even DOWN to 1.0.
+
+        This is the exact shape that breaks a shortest-then-round
+        strtod that truncates its mantissa buffer without carrying a
+        "non-zero was dropped" flag through to the tie-break — the
+        truncated form looks like an exact halfway and rounds the
+        wrong way. fast_float's Eisel-Lemire + digit-comparison
+        bignum fallback handles this correctly.
+        """
+        halfway = "1.00000000000000011102230246251565404236316680908203125"
+        s = halfway + "0" * 800 + "1"
+        self.assertEqual(float(s), 1.0 + 2 ** -52)
+
 if __name__ == "__main__":
     unittest.main()