fix(train): Enforce Nova Restricted storage on all MTRL MPG branches

[SifeiLi]

Restricted managed storage was only applied when MultiTurnRLTrainer auto-created a fresh model package group. The explicit output_model_package_group branch and the continued-customization branch (group derived from the source ModelPackage) performed no Nova check.

Centralize the requirement in _nova_managed_configuration() and apply it on every resolution path for both the output and intermediate checkpoint groups. Existing groups that do not satisfy the requirement now fail fast at trainer construction with an actionable error, since groups cannot be converted after creation. Non-Nova (OSS) models are unaffected.

Adds unit tests for the explicit, object, derived, and pre-existing default-named group branches, plus OSS pass-through.

Issue #, if available:

Description of changes:

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Tested below cases

Integration Tests — Happy Path (real SageMaker, us-east-1, wheel build)

#	Case	Verification Method	Result
I1	Nova + explicit Standard group	Trainer construction raises ValueError (bug repro — old SDK proceeded silently)	✅ PASS
I2	Nova + no group (auto-create)	DescribeModelPackageGroup on both auto-created groups returns ManagedStorageType=Restricted (output + intermediate checkpoint)	✅ PASS
I3	Nova + explicit Restricted group	Accepted, resolves to group ARN	✅ PASS

Integration Tests — Deny Cases (real SageMaker, us-east-1, wheel build)

#	Case	Why It Matters	Result
D1	Explicit Standard output group (name string)	Original repro path	✅ PASS
D2	Pre-existing default-named Standard group (nova-…-mtrl-mpg)	Residue left by the old buggy SDK in customer accounts is not silently reused	✅ PASS
D3	Explicit Standard intermediate checkpoint group	Second resolver enforced, not just output	✅ PASS
D4	ModelPackageGroup object referencing Standard group	Caller-constructed objects cannot bypass validation (authoritative re-fetch)	✅ PASS
D5	Group derived from source ModelPackage (continued customization)	The exact branch from the issue — Nova chains no longer inherit Standard groups	✅ PASS

Notes: All rejections produce the actionable message "ModelPackageGroup 'X' uses 'Standard' managed storage, but model 'Y' requires 'Restricted'. Existing groups cannot be converted…". Integration runs used the built wheel
(sagemaker_train-1.20.0) in a clean venv; construction exercised the full real path (live hub metadata for nova-textgeneration-lite-v2, real MPG describe/create, S3 validation). All test fixtures deleted post-run. Non-Nova (OSS)
models: no validation, no extra API calls — behavior unchanged.