Enhance tests and CI configuration for builder with linting

[rashmi-tondare]

This PR addresses issue #69

Fix tests & add CI
Make the test suite runnable without a live Kubernetes cluster by adding a mock HTTP server fallback. Tests now check for a kubectl proxy at 127.0.0.1:8001 and automatically use an in-process mock when it's unavailable.

Also adds:

• golangci-lint configuration (.golangci.yml)
• GitHub Actions CI workflow (.github/workflows/ci.yml) that runs lint and tests on every PR
• Minor code formatting fixes flagged by the linter

[rashmi-tondare]

@sercand I suppose you will need to manually approve the first run of the CI workflow so that it can run and the checks will show up in this and subsequent PRs to the master branch.

[rashmi-tondare]

@sercand if the workflow changes look fine to you looks like you need to manaully approve the workflow specifically for it to run the first time. Unfortunately approving the PR does not trigger the workflow run. Once you approve the workflow and it runs successfully, the PR can be merged.
From the github docs - https://docs.github.com/en/actions/how-tos/manage-workflow-runs/approve-runs-from-forks

[sercand]

@rashmi-tondare when you submitted the PR the GitHub actions was disabled for this repo therefore the workflow approval section didn't appear 😄 If you send one more commit it will appear probably.