sdk/python: renew the relay reservation before the router drops it

[aojea]

Both probe CronJobs on bananas failed from 19:30 UTC today (Testnet Health run 36052804226). Every failed probe died dialing the python canary through the router with NO_RESERVATION (204).

go-libp2p's relay grants a reservation for one hour (ReservationTTL) and garbage-collects it on expiry. The Python SDK reserved once in join and never again, so one hour after each rollout the python canary became unreachable through the router while still advertising the relayed address, until the next rollout restarted it. The JS SDK is unaffected: js-libp2p's circuit listener renews on its own. It did not show up before because the SDK canaries landed today and bananas was redeployed twice within the hour.

• MeshSession._reservation_loop reserves again two minutes before the expiry the router returned, the lead go-libp2p's own autorelay uses. A router that closed the connection forgets the admission, so a renewal that finds the router disconnected dials and authenticates again first. A failed renewal is retried; the relay keeps the old reservation until it expires.
• join(reservation_lead=...) exposes the lead; the retry reuses refresh_retry.
• Test: the fake router records handshakes and RESERVEs and grants a short TTL. The new test sees a renewal on the same admission, then has the router drop the member and sees the next renewal run the handshake first.
• README: why this is in the SDK. py-libp2p 0.7's relay client cannot talk to a go-libp2p router (framing, already documented), and its RelayDiscovery re-reserves only after expiry, when the slot is already gone.

Not related to #498: the relay limit is read when a circuit opens; the reservation TTL path is untouched, and the Go canaries kept passing.

[gemini-code-assist]

Code Review

This pull request implements automatic relay reservation renewal and re-authentication for the Python SDK. It introduces a background reservation loop in MeshSession that monitors reservation expirations and triggers renewal before they lapse, re-dialing and re-authenticating if a router has disconnected. An integration test is added to verify this behavior. Feedback is provided to handle failures during the renewal process by disconnecting from the router, preventing a potential stuck state where subsequent retries skip dialing and authentication.