backend: portforward: Fix context key to match rest of backend

[r0hansaxena]

Summary

Port forwarding broke in 0.43 with "Unreachable" for some clusters. The port forward handler builds the cluster lookup key differently from the rest of the backend, so in some setups it looks for the cluster under a key that was never stored and fails.

This PR changes the port forward handlers to use the same key logic as the rest of the backend instead of building the key themselves.

Related Issue

Fixes #6069

Steps to Test

1. Use a cluster setup where port forwarding currently fails with "Unreachable"
2. Try to port forward a pod or service

I reproduced the issue locally and confirmed the same request fails before the change and succeeds after.

Notes for the Reviewer

The existing port forward tests were updated for the new function signatures.

[k8s-ci-robot]

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[r0hansaxena]

I'll address the failing check