feat(functions): wire get_by_id and update; correct three doc claims

[JosteinGj]

The api serves GET /functions/{id} and POST /functions/update; neither was in the SDK. Both are now wired, in the async client, the blocking mirror and the Python bindings.

update returns GraphDataWrapper<Node> rather than a flat Resource, because that is what the endpoint echoes and because the flat shape cannot represent a function at all: Resource requires isRoot, which a function does not have, so a flat echo fails to deserialize rather than merely losing fields.

What functions genuinely lack is /byids, /filter and /search — the only node type missing all three. by_ids stays a client-side filter over the listing and its doc now says so rather than calling it a gap "yet".

Three Function doc claims were wrong:

• related_resources said it was "populated server-side by FunctionService .list()". FunctionTransformer applies the shared node fields and the labels and never joins the edges in, so it is empty on list, on get_by_id and on the create echo — which, unlike /resources/create, is no exception, because it re-reads the rows through that same transformer.
• name is Option here but the api declares it non-null.
• delete answers 204 with no body, so the wrapper is always empty.

The live round-trip drives the two new endpoints and asserts the echo is a Node::Function. Its comment also no longer claims it needs a forecast-ema model template — that went away with the functions feature itself.