artifacts: parent lineage, duck-typed declaration protocol, content-hash versions

- Metadata gains parents (ordered ArtifactParent | bare-version entries,
  serialized to the new repeated parent_artifacts on both ProducedArtifact
  and ArtifactSpec) and version_from_content (opt-in: an empty version
  resolves to the literal's content hash at conversion/create time, making
  republish of identical content idempotent).
- Output conversion duck-types get_flyte_metadata() on top-level outputs so
  offloaded-asset types outside flyte.io (e.g. plugin volumes) can declare
  themselves; a new current_output_declares_artifact contextvar tells the
  value's transformer when a declaration is actually being emitted.
- ensure_artifactable and Artifact.create accept protocol-carrying values;
  create() gains parents= and seeds defaults from duck metadata like it does
  from the wrapper.
- flyteidl2 pinned to 2.0.48 (repeated parent_artifacts), incl. rs_controller.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N415hnSCNmZ2e7WrwEPJo7
Signed-off-by: Haytham Abuelfutuh <haytham@union.ai>

Author: EngHabu