Fix BatchEndpoint.defaults regression: restore attribute-access object from get()

[ayushhgarg-work]

What

Follow-up to #47497. That PR fixed BatchEndpoint.defaults snake_case serialization, but
also changed batch_endpoints.get() to return endpoint.defaults as a dict/None instead
of a BatchEndpointDefaults object. This broke the published batch sample notebooks, which
do:

endpoint = ml_client.batch_endpoints.get(name)
endpoint.defaults.deployment_name = deployment.name
ml_client.batch_endpoints.begin_create_or_update(endpoint).result()

On an endpoint with no defaults, endpoint.defaults became None, so
endpoint.defaults.deployment_name = ... raised:
AttributeError: 'NoneType' object has no attribute 'deployment_name'.
8 batch-endpoint samples failed on the release branch.

Fix

• _from_rest_object again returns the BatchEndpointDefaults object (attribute get/set works).
• _schema/.../batch_endpoint_defaults.py @post_load again returns the object.
• Kept the serialization fix in _to_rest_batch_endpoint (dict → camelCase wire format), which
  is what actually resolved BatchEndpoint defaults serialization regression - deployment_name not converted to camelCase on wire #47478. Round-tripping still serializes correctly.

Tests

• Replaced the test that asserted the dict/None behavior with two regression guards:
  attribute access from get(), and the full sample flow (mutate .deployment_name → re-serialize
  → asserts {"deploymentName": ...}).
• 11 passed in tests/batch_online_common/unittests/test_endpoint_entity.py.

[Copilot]

Pull request overview

This PR addresses a regression in azure-ai-ml where ml_client.batch_endpoints.get() could return BatchEndpoint.defaults as None/dict, breaking existing sample and user code that relies on attribute access (e.g., endpoint.defaults.deployment_name = ...). The intent is to restore an attribute-accessible defaults object while keeping correct camelCase serialization on the wire.

Changes:

• Restores BatchEndpoint.defaults to be an attribute-accessible object after _from_rest_object (instead of dict/None).
• Updates the batch endpoint defaults marshmallow schema to return a BatchEndpointDefaults object post-load.
• Replaces the prior dict/None test assertion with regression tests for attribute access and camelCase round-tripping.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
sdk/ml/azure-ai-ml/azure/ai/ml/entities/_endpoint/batch_endpoint.py	Changes _from_rest_object behavior for defaults to preserve attribute access.
sdk/ml/azure-ai-ml/azure/ai/ml/_schema/_endpoint/batch/batch_endpoint_defaults.py	Adjusts schema post-load to construct a BatchEndpointDefaults object.
sdk/ml/azure-ai-ml/tests/batch_online_common/unittests/test_endpoint_entity.py	Updates unit tests to guard attribute access + correct camelCase serialization round-trip.
sdk/ml/azure-ai-ml/CHANGELOG.md	Updates changelog entry to reflect the restored attribute-access behavior for get().