fix(server): avoid duplicate relay opens on reconnect

[drew]

Summary

Avoid sending a queued relay open twice during supervisor session setup while preserving replay to a replacement session. Clean restack of #3588 on the preceding relay fix.

Related Issue

No issue required: localized correctness follow-up to #3583.

Changes

• Track the session most recently sent each pending relay open.
• Skip duplicate delivery to that same session and add a regression test.
• Document the per-session delivery invariant.

Testing

• Original focused relay tests (3) and server suite (1,818 passed) passed in fix(server): avoid duplicate relay opens on reconnect #3588.
• mise run pre-commit passed on this restacked branch.
• Combined mise run ci fails in Go gateway-list tests because this host has /etc/openshell/gateways/default; see test(go): isolate gateway listing from host config #3590.

Checklist

• Conventional, signed-off commit.
• Architecture documentation updated.

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[elezar]

Cross-reference: #4011 has been approved and submitted to the merge queue. It addresses the same per-session duplicate relay delivery fixed in #3588 and restacked here, and strengthens the implementation by reserving queue capacity and atomically validating the session, recording delivery, and enqueueing the relay open.

Draft #3766 also carries this server fix plus deterministic reconnect conformance coverage. After #4011 merges, rebase #3766 and remove its overlapping server patch while retaining its tests and architecture documentation.