Don't send MTUD probes on unvalidated paths #175
Description
To reproduce:
cargo test --package quinn-proto --lib -- tests::migration --exact --nocapture
2025-11-23T09:39:42.415171Z TRACE server:recv{space=Data pn=10}: quinn_proto::connection: migration initiated remote=127.0.0.1:44434
2025-11-23T09:39:42.415228Z TRACE server:send{space=Data pn=8}: quinn_proto::connection: validating previous path with PATH_CHALLENGE 8359a0a437d94e49
2025-11-23T09:39:42.415249Z TRACE server:send{space=Data pn=8}: quinn_proto::connection::packet_builder: PADDING * 1165
2025-11-23T09:39:42.415301Z TRACE server:send{space=Data pn=9}: quinn_proto::connection: PATH_CHALLENGE d766b4fe81679039
2025-11-23T09:39:42.415322Z TRACE server:send{space=Data pn=9}: quinn_proto::connection: RETIRE_CONNECTION_ID sequence=1
2025-11-23T09:39:42.415343Z TRACE server:send{space=Data pn=9}: quinn_proto::connection::packet_builder: PADDING * 1163
2025-11-23T09:39:42.415377Z TRACE server: quinn_proto::connection: sending 1200 bytes in 1 datagrams
2025-11-23T09:39:42.415409Z TRACE server:send{space=Data pn=10}: quinn_proto::connection::packet_builder: PADDING * 1298
2025-11-23T09:39:42.415449Z TRACE server: quinn_proto::connection: writing MTUD probe probe_size=1326 self.path.remote=127.0.0.1:44434
2025-11-23T09:39:42.415463Z TRACE server: quinn_proto::connection: sending 1326 bytes in 1 datagrams
(Note the MTUD probe on the migrated path)