fix: resolve priority starvation in HandshakeEventStream poll_next

Changes the poll_next implementation to use an internal loop pattern
instead of returning Poll::Pending immediately after high-priority work.
This ensures all priority levels get fair polling opportunities.

The previous implementation would wake and return Pending after processing
high-priority channels, starving lower priorities. Now we use 'continue'
to loop back and check all priorities before returning Pending.

This fixes the fairness issue discovered during #1932 debugging where
mesh connectivity would establish but connections would disappear.

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: iduartgomez