mining: enforce minimum reserved weight for IPC

Previously a lower value was silently clamped to MINIMUM_BLOCK_RESERVED_WEIGHT.

test/functional/interface_ipc_mining.py

@@ -257,6 +257,15 @@ class IPCMiningTest(BitcoinTestFramework):
                 empty_block = await mining_get_block(empty_template, ctx)
                 assert_equal(len(empty_block.vtx), 1)
 
+                self.log.debug("Enforce minimum reserved weight for IPC clients too")
+                opts.blockReservedWeight = 0
+                try:
+                    await mining.createNewBlock(opts)
+                    raise AssertionError("createNewBlock unexpectedly succeeded")
+                except capnp.lib.capnp.KjException as e:
+                    assert_equal(e.description, "remote exception: std::exception: block_reserved_weight (0) must be at least 2000 weight units")
+                    assert_equal(e.type, "FAILED")
+
         asyncio.run(capnp.run(async_routine()))
 
     def run_coinbase_and_submission_test(self):