Review follow-ups on MqttConnection (Task 3):
- Every certificate test asserted rejection, so the accept branch was
unreachable-by-regression. Adds a leaf genuinely issued by the pinned CA and
asserts acceptance.
- ValidateAgainstPinnedCa never seeded ChainPolicy.ExtraStore from the incoming
chain, so a leaf behind an intermediate delivered during the handshake failed
despite a legitimate path to the pinned root. Seeds from both the incoming
chain's elements and its ExtraStore; CustomRootTrust still means only the
pinned roots may terminate the chain.
- A DisposeAsync racing an in-flight connect escaped as an unclassified
exception; it now folds into ObjectDisposedException.
- Promotes the single-caller concurrency invariant into the type remarks, with
the accurate blast radius (a leaked live connection, not a benign throw).
Serialising the lifecycle remains Task 4's job.
- X509Chain.Build can throw; an exception escaping a TLS validation callback is
an opaque handshake crash, so it is caught and refused.
- Adds a connect-retry test (Task 4's reconnect loop reuses the instance) and a
disposed-then-connect test.
Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
Joseph Doherty