[yang-models]: Add BackEndSpineRouter device type in yang model

[kperumalbfn]

Why I did it

Add a new device type in DEVICE_METADATA yang model

Work item tracking

38921303

How I did it

Updated the existing Sonic device_metadata yang model to include the new device type.

How to verify it

Added yang model tests for the new type.

[mssonicbld]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines:
There may be pipelines that require an authorized user to comment /azp run to run.

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).

[Copilot]

Pull request overview

This PR updates the sonic-device_metadata YANG model to recognize a new DEVICE_METADATA.type value, BackEndSpineRouter, and adds/updates YANG model validation test vectors to ensure the new device role is accepted.

Changes:

• Added BackEndSpineRouter to the allowed DEVICE_METADATA.type pattern in sonic-device_metadata.yang.
• Added a corresponding positive test case definition in the YANG model tests metadata.
• Added a corresponding test input payload in the YANG model tests config JSON.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/sonic-yang-models/yang-models/sonic-device_metadata.yang	Extends DEVICE_METADATA.type validation pattern to allow BackEndSpineRouter.
src/sonic-yang-models/tests/yang_model_tests/tests/device_metadata.json	Adds a new test case entry for validating BackEndSpineRouter as an accepted type value.
src/sonic-yang-models/tests/yang_model_tests/tests_config/device_metadata.json	Adds the corresponding test configuration payload using "type": "BackEndSpineRouter".