Fix batchendpoint defaults serialisation

[ayushhgarg-work]

Description

Since azure-ai-ml 1.33.0, setting the default deployment on a batch endpoint failed:

endpoint.defaults = {"deployment_name": "my-deployment"}
ml_client.batch_endpoints.begin_create_or_update(endpoint)
# HttpResponseError: Could not find member 'deployment_name'
# on object of type 'BatchEndpointDefaults'.

Root cause: the TypeSpec-generated client stores raw dicts verbatim, so the snake_case key reached the wire instead of deploymentName. When the endpoint had no prior defaults, the call silently no-op'd.

Fix

• entities/_endpoint/batch_endpoint.py — wrap dict into BatchEndpointDefaults(**...) in _to_rest_batch_endpoint; unwrap back to a snake_case dict in _from_rest_object.
• _schema/_endpoint/batch/batch_endpoint_defaults.py — YAML @post_load returns a plain snake_case dict for consistency.
• tests/batch_online_common/unittests/test_endpoint_entity.py — 3 regression tests (camelCase on wire, None stays None, round-trip stays snake_case).
• CHANGELOG.md — entry under 1.34.0.

Validation

• 121 passed, 2 skipped in batch unit tests.
• Live round-trip against sdkv2-20250424-ws: before: None → after: {'deployment_name': 'test-deployment-8'}.

[Copilot]

Pull request overview

This PR fixes a regression in azure-ai-ml where BatchEndpoint.defaults supplied as a user dict was being sent to the service with snake_case keys (e.g., deployment_name) instead of the expected wire-format camelCase (deploymentName), causing begin_create_or_update to fail or silently no-op in some cases.

Changes:

• Wrap BatchEndpoint.defaults dicts into the generated REST model during _to_rest_batch_endpoint serialization so the model emits correct camelCase on the wire.
• Normalize REST → entity deserialization to expose BatchEndpoint.defaults consistently as a snake_case dict.
• Add regression unit tests and document the fix in the changelog.

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	Converts defaults dicts to/from generated REST model to ensure correct wire casing and consistent public shape.
sdk/ml/azure-ai-ml/azure/ai/ml/_schema/_endpoint/batch/batch_endpoint_defaults.py	Adjusts schema post-load to return a plain dict (snake_case) instead of a REST model instance.
sdk/ml/azure-ai-ml/tests/batch_online_common/unittests/test_endpoint_entity.py	Adds unit tests validating wire camelCase serialization and snake_case round-trip behavior.
sdk/ml/azure-ai-ml/CHANGELOG.md	Adds a bugfix entry documenting the regression and behavior change.