test(serve): add ModelBuilder.register framework attribute default tests

[aviruthen]

Issue #, if available:

Description of changes:

Add regression tests verifying a freshly constructed ModelBuilder exposes
framework and framework_version defaulted to None, so register() does not raise
AttributeError when the user never sets them.

Closes #5391

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[github-actions]

Claude finished @aviruthen's task in 2m 25s —— View job

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Code Review — test(serve): ModelBuilder.register framework defaults

• Read PR diff and the referenced source (sagemaker-serve/src/sagemaker/serve/model_builder.py)
• Verify test targets match the real register() implementation
• Assess correctness, conventions, and coverage

Summary

This is a tests-only PR and it looks clean. It adds a focused regression test for #5391, locking in that a freshly constructed ModelBuilder exposes framework/framework_version defaulted to None so register() doesn't raise AttributeError.

I verified the tests against the source and they exercise the intended behavior accurately:

• Root-cause guard — _initialize_defaults sets both attributes to None at construction (model_builder.py:542-545). test_framework_attributes_default_to_none_on_construction asserts exactly this.
• End-to-end path — with model_package_group_name set and the default (non-PROPRIETARY) model_type, register() builds a single-container container_def and passes self.framework / self.framework_version into update_container_with_inference_params (model_builder.py:6178-6185). test_register_does_not_require_user_set_framework_attributes captures both values there, confirming they were read (as None), not missing.
• Decorator handling is anticipated correctly — the test pins session.sagemaker_config = {} (needed by the @_telemetry_emitter wrapper) and uses a plain MagicMock session so the @runnable_by_pipeline decorator takes the non-pipeline branch and returns the ARN (model_builder.py:6232-6242). Good attention to detail.
• Patching resolve_and_validate_role keeps construction offline (matches model_builder.py:530), consistent with how the SDK avoids IAM work when a role isn't supplied.

Minor observations (non-blocking)

• The tests depend on several internal patch targets (create_model_package_from_containers, get_model_package_args, update_container_with_inference_params, _prepare_container_def). That's expected for a unit test at this layer, but it does couple the test to register()'s internal structure — a future refactor of that method could require updating these patches. The docstrings already explain the intent well, which mitigates this.
• Consider a brief comment noting that the empty get_model_package_args return ({}) is only safe because create_model_package_from_containers is also mocked — otherwise a reader might expect real package args to flow through.

Neither is worth blocking on. No correctness, security, or performance concerns — the PR adds no production code and the tests are well-scoped and accurate.