An attached client re-acquires its runtime once its spawner exits

[abonneth]

Made with Cursor

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Note

Medium Risk
Changes local runtime lifecycle, session create paths, and runtime identity auth wiring; mistakes could mis-route sessions or accept the wrong process on a port, though behavior is covered by new tests.

Overview
When a local client attaches to a shared runtime whose spawner/watchdog has exited, it can be left talking to a dead port or a squatter. This PR adds RuntimeHold so sync/async Client.local keeps a durable handle (launch recipe + ownership) and can detect a lost runtime and re-attach at the same base URL before new work.

Client / AsyncClient now read local_runtime and ownership through _hold instead of storing a fixed LocalRuntime. Local sessions clients take the hold, call reacquire / reacquire_async before create_session, and bind remote cancel to the runtime of the moment. Runtime identity HTTP clients accept a callable token so bearer auth and HMAC proofs follow a replacement runtime’s new API key without rebuilding the whole client.

Adds an integration test that shuts down the mock runtime, starts a replacement on the same port with a new token, and asserts the next session create uses the fresh credentials.

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