feat(tests): port stPreCompiledContracts/modexp 128-byte carry cases (#2753)

Add two MODEXP parametrizations with 128-byte (1024-bit) operands to
test_modexp. Both use modulus 2**1024 - 0x69 and reduce to 9^exp mod m
(in the first case, base = m + 9). The modulus size forces the
implementation through the multi-word Montgomery/Barrett reduction
path rather than any 256-bit specialization.

Also bump the test's gas_limit from 500k to 2M so the Byzantium
pre-EIP-2565 cost (~682k for a 128-byte modulus) fits. The bump is a
no-op for every existing case: the "out-of-gas" params OOG on
modexp's own gas formula (huge declared exp/mod lengths), not on the
tx cap, so they stay OOG at 2M.

Ported from two legacy fillers that the bulk static-filler import in
#1442 did not pull in:

- src/LegacyTests/Constantinople/GeneralStateTestsFiller/
  stPreCompiledContracts/modexp_37120_37111_37111_1000000Filler.json
- src/LegacyTests/Constantinople/GeneralStateTestsFiller/
  stPreCompiledContracts/modexp_9_37111_37111_1000000Filler.json

Those fillers live in the ethereum/tests LegacyTests submodule
(ethereum/legacytests), which the original port did not descend into.

Author: chfast

tests/byzantium/eip198_modexp_precompile/test_modexp.py

@@ -449,11 +449,40 @@
             ),
             id="truncated_input_4",
         ),
+        # 128-byte (1024-bit) operands exercising the multi-word
+        # Montgomery/Barrett reduction carry path. Modulus is
+        # 2**1024 - 0x69, so neither operand hits any 256-bit
+        # specialization. Both cases reduce to 9^exp mod m (base=m+9 in
+        # the first case), and with this exp, 9^exp ≡ 9 mod m.
+        pytest.param(
+            ModExpInput(
+                base="ff" * 127 + "a0",
+                exponent="ff" * 127 + "97",
+                modulus="ff" * 127 + "97",
+            ),
+            ModExpOutput(returned_data="0x" + "00" * 127 + "09"),
+            id="modexp_37120_37111_37111_1000000",
+        ),
+        pytest.param(
+            ModExpInput(
+                base="09",
+                exponent="ff" * 127 + "97",
+                modulus="ff" * 127 + "97",
+            ),
+            ModExpOutput(returned_data="0x" + "00" * 127 + "09"),
+            id="modexp_9_37111_37111_1000000",
+        ),
     ],
     ids=lambda param: param.__repr__(),  # only required to remove parameter
     # names (input/output)
 )
 @pytest.mark.eels_base_coverage
+@pytest.mark.ported_from(
+    [
+        "https://github.com/ethereum/legacytests/blob/master/src/LegacyTests/Constantinople/GeneralStateTestsFiller/stPreCompiledContracts/modexp_37120_37111_37111_1000000Filler.json",
+        "https://github.com/ethereum/legacytests/blob/master/src/LegacyTests/Constantinople/GeneralStateTestsFiller/stPreCompiledContracts/modexp_9_37111_37111_1000000Filler.json",
+    ],
+)
 def test_modexp(
     state_test: StateTestFiller,
     mod_exp_input: ModExpInput | Bytes,
@@ -504,7 +533,7 @@ def test_modexp(
         ty=0x0,
         to=account,
         data=mod_exp_input,
-        gas_limit=500_000,
+        gas_limit=2_000_000,
         protected=True,
         sender=sender,
     )