fix(core): poll, attach and log transform jobs through the session's region

[AkshayShah03]

Issue #, if available: N/A (same class of bug as #5796, fixed for processing jobs in #6369)

Description of changes:

Transformer doesn't carry its session's region through to the transform job:

• transform() builds latest_transform_job = TransformJob(**transformed) without a client context. wait(), refresh() and stop() then use the process-default boto3 session and region. With a session in another region, the job is created there but polled in the default region.
• attach() passes the SageMaker Session to TransformJob.get(session=...), which expects a boto3 Session, so Transformer.attach(name, sagemaker_session=...) always fails with ValidationError: session Input should be an instance of Session.
• logs_for_transform_job has the same problem when describing the job through TransformJob.get(...).__dict__. It then reads PascalCase keys (TransformResources, TransformJobStatus) from that snake_case dict.

This change:

• binds the job to the session's boto session and region with _set_client_context;
• passes session=sagemaker_session.boto_session, region=sagemaker_session.boto_region_name to TransformJob.get in attach;
• describes the job through sagemaker_session.sagemaker_client.describe_transform_job in logs_for_transform_job, matching fix(processing): respect session region when waiting for processing jobs #6369.

The generated TransformJob.wait(logs=True) builds its CloudWatch log handler without a session, so log streaming there still uses the default region. That's generated code, so I left it alone.

Testing:

• test_transform_binds_job_to_session_region
• test_attach_uses_boto_session_and_region
• test_logs_for_transform_job_describes_with_session_client
• All three fail before this change. tests/unit/test_transformer.py: 33 passed.

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