[Bug]: history length is not optional in .proto

[yarolegovich]

What happened?

Description

Both fields are optional in types.ts and jsonrpc schema.
For both cases it might be useful to differentiate 0 and unset. For example:

1. A client calls GetTask and doesn't want History to be fetched / included in the response.
2. A client calls SendMessage and doesn't want history to be persisted at all.

The change is backward-compatible and makes it possible for a2a servers to provide consistent behavior regardless of the selected transport.

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