Replace per-field message fetch state with composable fetch items

[stevebauman]

Message fetch selection currently requires a boolean property, state getter, fluent inclusion method, fluent exclusion method, and command-builder condition for every supported data item. This makes each new FETCH item expand the public query contract and spreads protocol knowledge across the query implementation.

This introduces a uniform MessageData API for composing requests through with(), without(), and only(). Fixed attributes are represented internally by enum values, while BODY sections are immutable objects that can be transformed with peek() without adding global query state or applying irrelevant options to other data items.

For example:

$folder->messages()->with(
    MessageData::flags(),
    MessageData::headers()->peek(),
);

All message data items implement the same FetchItem contract, so the query only needs to serialize the selected items. FETCH response parsing and message construction now live in FetchedMessageData, keeping MessageQuery focused on search, ordering, pagination, and execution.