test(client): model SessionMachine open-session operation sequences

Adds a proptest-based model test that drives generated sequences of
probe sends, valid/duplicate/late/unknown/wrong-token replies, bounded
and unbounded timeout polling, peer/local close, and u32 wire-sequence
wraparound through a real SessionMachine and an independent reference
model, asserting the observable state and every emitted event match
after each operation. This combinatorial interleaving of an open
session's operations was previously untested; existing tests are all
narrow single-scenario cases.

crates/irtt-client/Cargo.toml

@@ -30,4 +30,5 @@ all-features = true
 
 [dev-dependencies]
 irtt-server.workspace = true
+proptest = "1.11.0"
 tokio = { version = "1.53.1", default-features = false, features = ["rt", "sync"] }