fix(aiosendspin adapters): update for encryption-era SendspinServer/SendspinClient constructors

[OnFreund]

Summary

• aiosendspin's encryption support (Sendspin-Protocol/aiosendspin#292 through #298) changed SendspinServer()/SendspinClient() to require an Identity keypair and a pairing store instead of a plain server_id/client_id string, and defaulted allow_unencrypted to False. The old constructor calls in both adapters raised a TypeError before server-ready.json/client-ready.json could be written, timing out every conformance scenario run with --from aiosendspin (or --to aiosendspin).
• Construct an Identity.generate() + in-memory pairing store on both aiosendspin_server.py and aiosendspin_client.py, and pass allow_unencrypted=True on the server so today's still-unencrypted client adapters continue to be admitted.
• Also fixed two smaller renames from the same aiosendspin release: client.negotiated_roles → client.active_role_ids, and the removed add_server_hello_listener callback → the existing client.server_info property.

Test plan

• Cloned current aiosendspin + sendspin-cli at HEAD into a scratch venv and ran the harness directly against this fix.
• Confirmed the server now logs Sendspin server started successfully and writes server-ready.json (previously: immediate TypeError crash).
• Hand-crafted an unencrypted legacy client/hello (mimicking untouched client SDKs like sendspin-jvm) and confirmed the server logs Accepting unencrypted legacy connection (transition mode) and returns a valid server/hello, validating the allow_unencrypted=True admission path called out in the issue.
• Ran a full aiosendspin↔aiosendspin conformance self-test: connection, Noise/legacy handshake, and role negotiation now succeed (previously impossible). Remaining media-transfer-level scenario failures (audio/metadata/artwork/FLAC negotiation) are further, separate protocol drift unrelated to this constructor crash — filed as a follow-up rather than folded into this fix.

Fixes #93

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