fix(serve): let ModelBuilder.register() omit content and response types

[Thanatos9404]

Issue #, if available: Fixes #6480

Description of changes:

ModelBuilder.register() takes content_types and response_types as optional arguments (default None). It sets self.content_types and self.response_types only when they are passed, but always reads both attributes for get_model_package_args(), so leaving either one out raised AttributeError: 'ModelBuilder' object has no attribute 'content_types' (or 'response_types').

This change defaults both attributes to None in _initialize_defaults(), next to framework and framework_version, which register() reads the same way and which were defaulted there for #5391 (#5424). get_model_package_args() already leaves None values out, so the request has no SupportedContentTypes or SupportedResponseMIMETypes when the types are not given. Both fields are optional in CreateModelPackage (InferenceSpecification requires only Containers). register() itself is unchanged, so types passed to an earlier register() call are still reused by a later call that leaves them out, as the issue asks.

Testing:

• New sagemaker-serve/tests/unit/test_model_builder_register.py: register() with neither argument, with only content_types, and with only response_types (all 3 fail on master with the AttributeError and pass with this change), plus a check that a second call reuses the types passed to the first.
• The reproduction from the issue, run offline (PipelineSession with dummy credentials, no AWS calls): on master it raises the AttributeError; with this change the captured CreateModelPackage request has no SupportedContentTypes or SupportedResponseMIMETypes.
• pytest tests/unit -m "not slow_test" in sagemaker-serve (Python 3.12, run before rebasing onto fix(serve): restore JumpStart default instance type from model spec #6045): no new failures compared with master; the only differences are the 3 new cases, which fail on master. After the rebase, the new tests and TestRegisterReturnsArn give 36 passed.
• black --check and flake8 pass on the changed files, pydocstyle passes on model_builder.py, and pylint with the repo .pylintrc reports the same messages for model_builder.py before and after the change.

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[Thanatos9404]

Closing this, since #6489 (merged) also fixes #6480: register() now reads content_types and response_types with getattr and has tests for omitting them. Thanks!