Add unit tests for attachablePodForObject function

[GautamBytes]

Added unit test for pkg/internal/polymorphichelpers/attachablepodforobject.go .

Our tests successfully validated all the logic paths that don't require a live Kubernetes connection:

• The case where the object is already a pod.
• The error path if the REST config fails.
• The error path if SelectorsForObject fails (e.g., for an unsupported type).

The uncovered code is the final "success path" where the function tries to:

• Create a real Kubernetes clientset.
• Use that clientset to find and retrieve a running pod.

This part of the code is not covered because a simple unit test cannot create a real Kubernetes client and connect to the API.

[kruise-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign hantmac for approval by writing /assign @hantmac in a comment. For more information see:The Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[GautamBytes]

@furykerry @FillZpp just pinging for review. Thanks!

[GautamBytes]

@furykerry @FillZpp just pinging for review. Thanks!

@furykerry PTAL!

[GautamBytes]

@furykerry @hantmac will you merge these test pr's please!