fix: harden Lease acquisition disconnect handling

[smiggleworth]

Summary

• allocate deferred Lease acquisition futures only for positive waits
• observe internal deferred failures immediately so disconnect cannot produce an unretrieved future exception
• remove stale waiters deterministically while preserving the public transport failure and cancellation behavior
• export Schedule ERR_BACKEND_ERROR (7010) and classify it as retryable for replay-safe operations

This is the Python parity companion to cntryl/fitz-ts#22. Stream envelopes and bounded subscription delivery already matched the intended behavior.

Validation

• Ruff format and lint
• Pyright
• 106 unit tests with branch coverage above 70 percent
• source distribution and wheel build
• 8 broker integration tests
• four-leg TCP/WebSocket and anonymous/valid-JWT conformance matrix

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