Fix bridge session expiry at startup before bridges are started

[v1rtu3x]

In 2.1.x, session_expiry__check() was added to broker startup and runs
before bridge__start_all(). This causes restored outgoing bridge sessions
to be incorrectly expired if persistent_client_expiration has passed,
silently discarding the bridge's offline message queue.

Add a bridge guard in session_expiry__check() to skip outgoing bridge
sessions, consistent with the existing 'Outgoing bridge connection never
expire' guard in context__disconnect().

Fixes #3728

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