fix(python): bump lease_rotate_interval_ms default 50 -> 250 to match Rust/docs

[Copilot]

Summary

The Python client still defaulted queue_storage_lease_rotate_interval_ms to 50, while the Rust default and docs moved to 250ms in #169 (and carried through #321). This drift meant Python users got a different lease-rotation cadence than the documented/Rust default unless they passed the argument explicitly.

This brings the Python bindings in line:

• awa-python/src/client.rs — pyo3 start signature default 50 → 250
• awa-python/python/awa/client.py — AsyncClient keyword default 50 → 250
• awa-python/python/awa/_awa.pyi — type stub default 50 → 250

Context

Ideally this would land on the open PR #321, but that PR's branch (issue-295-v06-sequence-cursors) isn't writable from this environment, so it's surfaced here as a standalone follow-up. It is a one-commit change stacked on top of #321's head.

Validation

• Confirmed all three Python binding sites now read 250 and no 50ms lease-rotate default remains.
• Cross-checked against the Rust default referenced in awa-worker/src/client.rs and awa-worker/src/maintenance.rs (250ms / Validate MVCC / dead-tuple behaviour under sustained load #169).