feat: validate kubectl context matches deploy.kubeContext config

[westoque]

Fixes: #9144
See: https://groups.google.com/g/skaffold-users/c/wWbavyfyd5w/m/GE0q6z5aBQAJ

Description
Prevent accidental deployments to wrong cluster by validating that the current kubectl context matches deploy.kubeContext in skaffold.yaml.

Validation is skipped when kubeContext is overridden via CLI or unset.

User facing changes (remove if N/A)
This will show a warning when running "skaffold dev" and not in the right context specified in deploy.kubeContext in skaffold.yaml

genoa@Williams-MacBook-Pro-2:~/GitHub/narralabs/microservices-base(master+*)[hetzner]$ /Users/genoa/.local/share/mise/installs/go/1.25.3/bin/skaffold dev
creating runner: creating deployer: kubectl context mismatch: current context is "hetzner" but deploy.kubeContext is set to "custom" in skaffold.yaml. This prevents accidental deployments to the wrong cluster. To deploy to "custom", switch your context with: kubectl config use-context custom

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[gemini-code-assist]

Summary of Changes

Hello @westoque, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances Skaffold's deployment safety by implementing a crucial kubectl context validation. It ensures that the active Kubernetes context aligns with the context defined in the skaffold.yaml for deployment, thereby mitigating the risk of deploying applications to the wrong cluster. The feature provides a clear warning to the user upon mismatch, while also allowing for intentional context overrides through command-line arguments.

Highlights

• Kubectl Context Validation: Introduced a new validation step to ensure the current kubectl context matches the deploy.kubeContext specified in skaffold.yaml.
• Accidental Deployment Prevention: This validation aims to prevent accidental deployments to unintended Kubernetes clusters by halting execution and providing a clear error message if a mismatch is detected.
• CLI Override Support: The validation is intelligently skipped if the kubeContext is explicitly overridden via a CLI flag or environment variable, respecting user intent.
• New Functionality: A new validateKubeContext function has been added to pkg/skaffold/runner/deployer.go and integrated into the GetDeployer function.
• Comprehensive Testing: Extensive unit tests have been added in pkg/skaffold/runner/deployer_test.go to cover various scenarios, including single and multiple pipeline configurations, and CLI overrides.

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[gemini-code-assist]

Code Review

This pull request introduces a valuable feature to prevent accidental deployments to the wrong Kubernetes cluster by validating the kubectl context. The implementation is clean and the added tests cover the main scenarios. I've identified a minor issue with the user-facing error message that could be confusing, and I've provided a suggestion to clarify it. Additionally, I've recommended an improvement to the tests to make them more robust by checking for the exact error message, which will help prevent similar issues in the future. Overall, this is a solid contribution.