Document server-default schedule catchup window

[chaptersix]

What changed

• Document that a null catch-up window is omitted and defaults to one year on the server.
• Add protobuf conversion coverage proving the field is absent unless explicitly configured.
• Add server-backed describe coverage for the one-year default.

Why

Java already serialized an unspecified value correctly, but the public contract and regression coverage were incomplete.

Breaking changes

None.