ModelBuilder.deploy(update_endpoint=True) raises AttributeError on Session.create_endpoint_config()

**PySDK Version**

- [ ] PySDK V2 (2.x)
- [x] PySDK V3 (3.x)

**Describe the bug**

`ModelBuilder.deploy(update_endpoint=True)` raises `AttributeError: 'Session' object has no attribute 'create_endpoint_config'`.

In the installed V3 packages, `ModelBuilder._deploy_core_endpoint()` calls `self.sagemaker_session.create_endpoint_config(...)` on the update path, but `sagemaker.core.helper.session_helper.Session` does not implement that method.

**To reproduce**

Run the following script:

```python
from types import SimpleNamespace

import boto3
from sagemaker.core.helper.session_helper import Session
from sagemaker.serve.model_builder import ModelBuilder

builder = ModelBuilder(
s3_model_data_url={
"S3DataSource": {
"S3Uri": "s3://example-bucket/model/",
"S3DataType": "S3Prefix",
"CompressionType": "None",
}
},
image_uri="123456789012.dkr.ecr.us-west-2.amazonaws.com/example:latest",
role_arn="arn:aws:iam::123456789012:role/SageMakerExecutionRole",
sagemaker_session=Session(boto_session=boto3.Session(region_name="us-west-2")),
instance_type="ml.g5.xlarge",
env_vars={},
)

builder.model_name = "demo-model"
builder.region = "us-west-2"
builder.endpoint_name = "demo-endpoint"
builder.built_model = SimpleNamespace(model_name="demo-model")

builder._deploy_core_endpoint(
endpoint_name="demo-endpoint",
instance_type="ml.g5.xlarge",
initial_instance_count=1,
wait=False,
update_endpoint=True,
)
```

**Steps**

1. Create a Python environment with the V3 SageMaker packages listed below.
2. Save the script above.
3. Run `python minimal_repro_model_builder_update_endpoint.py`.

**Expected behavior**

One of the following should happen:

- `ModelBuilder.deploy(update_endpoint=True)` should work with the default `Session` implementation used by V3 packages.
- Or `ModelBuilder` should use a session method that actually exists on `sagemaker.core.helper.session_helper.Session`.

**Screenshots or logs**

Actual error:

```text
AttributeError: 'Session' object has no attribute 'create_endpoint_config'
```

I also confirmed locally that:

- `hasattr(Session, "create_endpoint_config") == False`
- `ModelBuilder._deploy_core_endpoint()` contains a call to `self.sagemaker_session.create_endpoint_config(...)`

In my real deployment code, this was triggered from `ModelBuilder.build(...); ModelBuilder.deploy(..., update_endpoint=True)` while updating an existing SageMaker endpoint.

**System information**

- **SageMaker Python SDK version**: V3 packages; `sagemaker-core==2.5.0`, `sagemaker-serve==1.5.0`
- **Framework name (eg. PyTorch) or algorithm (eg. KMeans)**: N/A for repro; issue is in `ModelBuilder` update flow
- **Framework version**: N/A
- **Python version**: 3.14.3
- **CPU or GPU**: Repro itself is local and does not require either; production usage was GPU-backed SageMaker endpoint deployment
- **Custom Docker image (Y/N)**: N

**Additional context**

- Avoiding `update_endpoint=True` and recreating the endpoint instead works around the issue.
- This looks related to the ongoing V3 API/session transition, and may be adjacent to issue #5336, but this report is specifically about the missing `create_endpoint_config()` method on the session object used by `ModelBuilder`.

Start by reproducing the failure with the minimal script, then inspect ModelBuilder._deploy_core_endpoint and sagemaker.core.helper.session_helper.Session, focusing on the update_endpoint path and the missing create_endpoint_config call. Confirm the fix by running the reproduction and verifying that updating an existing endpoint works with the default V3 Session implementation.

Written by the indexing model from the issue text.