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/interface_ipc_mining.py

@@ -7,7 +7,6 @@ import asyncio
 import time
 from contextlib import AsyncExitStack
 from io import BytesIO
-import platform
 from test_framework.blocktools import NULL_OUTPOINT
 from test_framework.messages import (
     MAX_BLOCK_WEIGHT,
@@ -42,6 +41,7 @@ from test_framework.ipc_util import (
     mining_wait_next_template,
     wait_and_do,
     make_mining_ctx,
+    assert_capnp_failed
 )
 
 # Test may be skipped and not have capnp installed
@@ -325,15 +325,7 @@ class IPCMiningTest(BitcoinTestFramework):
                 await mining.createNewBlock(ctx, opts)
                 raise AssertionError("createNewBlock unexpectedly succeeded")
             except capnp.lib.capnp.KjException as e:
-                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_equal(e.description, "remote exception: std::exception: block_reserved_weight (0) must be at least 2000 weight units")
-                assert_equal(e.type, "FAILED")
+                assert_capnp_failed(e, "remote exception: std::exception: block_reserved_weight (0) must be at least 2000 weight units")
 
         asyncio.run(capnp.run(async_routine()))
 
@@ -357,6 +349,13 @@ class IPCMiningTest(BitcoinTestFramework):
                 block.hashMerkleRoot = block.calc_merkle_root()
                 original_version = block.nVersion
 
+                self.log.debug("Submit solution that can't be deserialized")
+                try:
+                    await template.submitSolution(ctx, 0, 0, 0, b"")
+                    raise AssertionError("submitSolution unexpectedly succeeded")
+                except capnp.lib.capnp.KjException as e:
+                    assert_capnp_failed(e, "remote exception: std::exception: SpanReader::read(): end of data:")
+
                 self.log.debug("Submit a block with a bad version")
                 block.nVersion = 0
                 block.solve()