Add a multi-server arbitration scenario

[balloob]

This test scenario was identified by a cross-SDK conformance audit comparing every Sendspin client/server implementation. See the audit docs on branch claude/stream-sync-correction-sdks-AWoNC.

What it tests

Two servers connect to one client (one with
connection_reason: 'discovery', one 'playback'); the harness
asserts the client sends client/goodbye with the right reason
('another_server') on the right socket and ends up speaking to the
right server per the spec's decision table.

Why we need it

The spec's multi-server arbitration rules ("must complete handshake
first, prefer last-played server, send another_server on switch")
are unenforced. Only sendspin-cpp and sendspin-cli implement
them; six SDKs collapse to "whichever connection happened first" and
default to the wrong goodbye reason on switch.

Implementation sketch

• Harness spins up two aiosendspin server adapters with different
  connection_reason values and a stable server_id per server.
• Client adapter is configured with one server as "last played".
• Harness asserts on the goodbye message contents (socket + reason)
  and on which server receives subsequent client/state updates.

Variants worth covering: (a) playback arrives while existing is
discovery, (b) both discovery and last-played matches one, (c)
both discovery and neither matches.

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Source: docs/reconnection-and-multi-server.md §recommendations