feat(assets): the typed /assets service

[JosteinGj]

The api grew a typed /assets family — the last node type without one. Every call is the shared /resources pipeline with the ASSET discriminator pinned server-side, so the two paths cannot drift apart on ACLs, naming policy or status codes. What differs is the shape that comes back: Asset rather than the polymorphic Node, so geolocation and is_root are reachable without a match.

AssetsService covers the whole surface — create, get_by_id, by_ids, list, filter, search, update, delete — in the async client, the blocking mirror and the Python bindings (sync and async).

filter and search reuse ResourceFilterForm and SearchAndFilterForm<ResourceFilter> rather than minting a second Rust spelling of one wire shape: the api declares the same Java types on both endpoint families. A nodeType in the body is replaced with ["asset"], not merged — entries OR together, so merging would widen a request to /assets into a mixed query. The Python filter therefore has no node_type keyword at all; offering a parameter the server silently discards is worse than omitting it.

Behaviours worth knowing, each pinned by a test:

• A 404 from get_by_id does not mean the id is free. A node of another type, and an asset you may not read, are both reported as missing.
• Unlike /resources/create, the caller never sets the ASSET label — the endpoint deserializes into the api's Asset, whose constructor forces it in.
• delete answers 204 with no body, so the wrapper is always empty.
• update echoes a typed Node, and geolocation is the one update field that means anything on exactly one node type.

PyAsset already existed in the bindings' nodes.rs — it is what PyNode dispatches an ASSET row to — so this adds the service pair only.