mining: add interrupt()

Both waitTipChanged() and createNewBlock() can take a long time to
return. Add a way for clients to interrupt them.

The new m_interrupt_mining is safely accessed with a lock on
m_tip_block_mutex, but it has no guard annotation. A more thorough
solution is discussed here:
https://github.com/bitcoin/bitcoin/pull/34184#discussion_r2743566474

test/functional/interface_ipc_mining.py

@@ -137,6 +137,14 @@ class IPCMiningTest(BitcoinTestFramework):
             assert_equal(oldblockref.hash, newblockref.hash)
             assert_equal(oldblockref.height, newblockref.height)
 
+            self.log.debug("interrupt() should abort waitTipChanged()")
+            async def wait_for_tip():
+                long_timeout = 60000.0  # 1 minute
+                result = (await mining.waitTipChanged(ctx, newblockref.hash, long_timeout)).result
+                # Unlike a timeout, interrupt() returns an empty BlockRef.
+                assert_equal(len(result.hash), 0)
+            await wait_and_do(wait_for_tip(), mining.interrupt())
+
         asyncio.run(capnp.run(async_routine()))
 
     def run_block_template_test(self):
@@ -163,6 +171,14 @@ class IPCMiningTest(BitcoinTestFramework):
                 # spurious failures.
                 assert_greater_than_or_equal(time.time() - start, 0.9)
 
+                self.log.debug("interrupt() should abort createNewBlock() during cooldown")
+                async def create_block():
+                    result = await mining.createNewBlock(ctx, self.default_block_create_options)
+                    # interrupt() causes createNewBlock to return nullptr
+                    assert_equal(result._has("result"), False)
+
+                await wait_and_do(create_block(), mining.interrupt())
+
                 header_only_peer.peer_disconnect()
                 self.connect_nodes(0, 1)
                 self.sync_all()