Defer schedule catchup window default to server

[chaptersix]

What changed

• Change the default SchedulePolicy.catchup_window from an SDK-defined year to None.
• Omit the protobuf field when it is unset.
• Preserve absent fields as None when decoding.
• Add serialization and server-backed default coverage.
• Update the changelog.

Why

The server owns the default. Materializing one year in the SDK prevents server-side default changes from applying consistently.

Validation

• Ruff lint and format checks
• Pyright: zero errors
• Runtime pytest was not run locally because Rust is unavailable for building the Python bridge.

Breaking changes

An unconfigured policy now exposes None before it is sent to the server. Described schedules continue to expose the server-resolved duration.

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[CLAassistant]

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