fix: resolve private hub Models and aliased references for modelTrainer

[tanvikab4]

Description of changes

Summary

ModelTrainer.from_jumpstart_config(...) could not resolve models that live in a private hub — only public
JumpStart models and (by a fragile assumption) plain private-hub references worked. This change fixes hub-content
resolution so ModelTrainer reaches parity with ModelBuilder, supporting:

• Public JumpStart models (unchanged)
• Model References in a private hub (pointer to a public model)
• Aliased references — filed under a hub content name that differs from the public model_id
• Privately-owned Models authored directly into a private hub

Root cause

sagemaker.core.jumpstart.document.get_hub_content_and_document() guessed the hub content type from the hub name:

hub_content_type = "Model" if hub_name == SAGEMAKER_PUBLIC_HUB else "ModelReference"

A private hub can hold either a Model or a ModelReference. This guess meant:

• Privately-owned Models were looked up as ModelReference → ResourceNotFound → resolution failed.
• The lookup used model_id and ignored hub_content_name, so aliased references were never found.

Fix (sagemaker-core/src/sagemaker/core/jumpstart/document.py)

• Replace the guess with a probe: for a private hub, try ModelReference first, then fall back to Model; the
  public hub uses Model only. This mirrors ModelBuilder's resolution in accessors.py.
• Honor hub_content_name (falling back to model_id) so aliased references resolve.
• On miss, raise a combined error naming both content types attempted.

No changes were needed elsewhere: defaults.py already attaches HubAccessConfig based on hub_content_type, and
model_trainer.py / JumpStartConfig already support hub_name/hub_content_name — they become correct automatically
once the content type is resolved honestly.

Testing

Unit (sagemaker-core/tests/unit/jumpstart/test_document.py) — 5 new tests, all passing:

• public hub resolves as Model (single lookup, no probe)
• private-hub reference resolves on the first probe
• private-hub Model resolves via the fallback (asserts probe order ["ModelReference", "Model"])
• hub_content_name alias is used for lookup
• neither type present → raises after attempting both

Integration (sagemaker-train/tests/integ/jumpstart/test_jumpstart_train.py) — 3 new tests, each creates a
temporary private hub, runs a real training job, and tears down (skips gracefully without hub permissions). All
verified passing end-to-end against AWS:

• test_jumpstart_train_from_private_hub_reference
• test_jumpstart_train_from_aliased_reference
• test_jumpstart_train_from_private_owned_model