test: synchronize replacement WebSocket handshake (#3891)

The reconnect shutdown regression could fail during cleanup because the
replacement client handshake completed and closed before the server
application handler recorded it. Wait for explicit server acceptance
before returning the replacement socket to the shutdown path.

The test retains both close-deadline variants, the two-connection
assertion, and reader/ownership cleanup checks. No SDK runtime,
dependency, or release metadata changes.

Validation: 156 session tests, both targeted variants, Ruff and Pyright
pass. The local mypy run reports two identical errors on the unchanged
baseline and candidate.

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>