test: verify IPC error handling for invalid coinbase

Add a test case to interface_ipc_mining.py to verify that the IPC
server correctly handles and reports serialization errors rather than
crashing the node.

This covers the scenario where submitSolution is called with data
that cannot be deserialized, as discussed in #33341

Also introduces the assert_capnp_failed helper in ipc_util.py to
cleanly handle macOS-specific Cap'n Proto exception strings, and
refactors an existing block weight test to use it.

test/functional/test_framework/ipc_util.py

@@ -10,9 +10,13 @@ from dataclasses import dataclass
 from io import BytesIO
 from pathlib import Path
 import shutil
+import platform
 from typing import Optional
 
 from test_framework.messages import CBlock
+from test_framework.util import (
+    assert_equal
+)
 
 # Test may be skipped and not have capnp installed
 try:
@@ -155,3 +159,14 @@ async def make_mining_ctx(self):
     self.log.debug("Create Mining proxy object")
     mining = init.makeMining(ctx).result
     return ctx, mining
+
+def assert_capnp_failed(e, description_prefix):
+    if e.description == "remote exception: unknown non-KJ exception of type: kj::Exception":
+        # macOS + REDUCE_EXPORTS bug: Cap'n Proto fails to recognize
+        # its own exception type and returns a generic error instead.
+        # https://github.com/bitcoin/bitcoin/pull/34422#discussion_r2863852691
+        # Assert this only occurs on Darwin until fixed.
+        assert_equal(platform.system(), "Darwin")
+    else:
+        assert e.description.startswith(description_prefix), f"Expected description starting with '{description_prefix}', got '{e.description}'"
+    assert_equal(e.type, "FAILED")