Artifact creation now uses idempotency keys to avoid duplicate creates on retry

What

• Artifact creation now computes and sends an idempotency key with the create-artifact request, guarding against duplicate creation when a call is retried.
• If a create is already in progress under the same key, the server now returns a create_in_flight error (HTTP 409) instead of creating a duplicate; new telemetry fields already_created and keyed are logged, and responses can include alreadyCreated: true for replayed creates.
• When an artifact-publish tool call result indicates the artifact was already created by an earlier attempt of the same call, the tool now returns "An earlier attempt of this call already created the Artifact..." instead of claiming a fresh creation.
• Similarly, if an Artifact-from-type creation succeeds but publishing its files fails, the resulting message now checks already_created and, if true, reports that nothing new was created instead of always saying a new Artifact was created.

Why

This prevents retried or duplicate artifact-creation calls, for example after a network hiccup, from producing confusing duplicate Artifacts or misleading "Created a new Artifact" messages.