JumpStartModel corrupts environment variables (incorrect snake_case conversion) when using a private hub

**PySDK Version**

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

**Describe the bug**
When using the `JumpStartModel` class to deploy a model reference hosted on a private hub (by passing the `hub_name` parameter), a subset of the container environment variable names arrive corrupted. The variable names from the instance-specific overrides are recursively converted from UpperCamelCase to snake_case. The SDK uses a regular expression that inserts an underscore before every capital letter and then lowercases the result.

As a result, a variable like `SM_VLLM_MAX_MODEL_LEN` becomes `s_m__v_l_l_m__m_a_x__m_o_d_e_l__l_e_n`. This causes the instance-specific override to silently fail and not merge over the base defaults, as the spelling collision is broken. The defect is located in the hub deserialization path (`JumpStartInstanceTypeVariants.from_describe_hub_content_response`), which applies a `walk_and_apply_json(response, camel_to_snake)` function across the entire document structure.

**To reproduce**

```python
from sagemaker.jumpstart.model import JumpStartModel

# Note: Requires a private hub setup with a ModelReference to a JumpStart model
model = JumpStartModel(
model_id="huggingface-vlm-gemma-4-e4b-it",
model_version="*",
config_name="generate_lowest_cost",
hub_name="mi-hub-jumpstart-2", # Replace with your private hub name
region="eu-south-2",
)

# Inspect the container definition before creating the model
container_def = model.prepare_container_def(
accept_eula=True
)

# Check the environment variables
print(container_def["Environment"])

```

The environment variables will already show up as corrupted when executing this method.

**Expected behavior**
The environment variables from instance-specific overrides (located under `HostingInstanceTypeVariants`) should retain their original names (e.g., `SM_VLLM_MAX_MODEL_LEN`). The values from these instance-specific overrides should properly override the base defaults without having their keys modified into snake_case with underscores during JSON parsing.

**Screenshots or logs**
Example of the corrupted environment block showing duplicated values:

```text
SM_VLLM_MAX_MODEL_LEN = 4096
s_m__v_l_l_m__m_a_x__m_o_d_e_l__l_e_n = 131072
h_f__h_u_b__o_f_f_l_i_n_e = 1

```

**System information**

* **SageMaker Python SDK version**: 2.245.0 (Defect confirmed by AWS Support to also be present in recent versions 2.257.6 and 3.20.0)

* **Framework name (eg. PyTorch) or algorithm (eg. KMeans)**: JumpStart

* **Framework version**: N/A
* **Python version**: Python 3

* **CPU or GPU**: Any

* **Custom Docker image (Y/N)**: N

**Additional context**
AWS Premium Support investigated this under Case ID 178704468000307. They confirmed that the equivalent public-hub function, `from_json`, performs no key conversion at all, which is why the exact same model deploys correctly when taken from the public catalog. The issue occurs specifically because the names in `HostingInstanceTypeVariants` are defined as keys in a map, and are thus incorrectly caught by the `camel_to_snake` recursive rewrite during hub deserialization.

Bắt đầu từ JumpStartInstanceTypeVariants.from_describe_hub_content_response và theo dõi lệnh gọi walk_and_apply_json(response, camel_to_snake) của nó. So sánh đường dẫn hub này với hành vi public-hub của from_json, sau đó tái hiện bằng JumpStartModel.prepare_container_def với một hub riêng tư. Hoàn tất khi các khóa như SM_VLLM_MAX_MODEL_LEN vẫn không thay đổi và các giá trị dành riêng cho instance ghi đè các giá trị mặc định cơ sở.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.