backend: Refactor createHeadlampHandler to reduce complexity

[SinghaAnirban005]

The createHeadlampHandler function was too long and complex making it
hard to maintain.

fixes #3273

Changes made -> Split logic into smaller helper functions where appropriate

[k8s-ci-robot]

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Pull Request Overview

Refactors createHeadlampHandler by extracting helper functions to improve readability and reduce complexity.

• Extracted server logging and configuration setup into logServerConfiguration and setup* functions.
• Modularized route definitions (metrics, proxy, OIDC, frontend) into dedicated setup* functions.
• Segregated proxy and OIDC handling into discrete functions for clarity.

Comments suppressed due to low confidence (3)

backend/cmd/headlamp.go:643

• New helper functions like getProxyURL, isURLAllowed, and getResponseReader lack unit tests. Adding tests for these will ensure correctness and prevent regressions.

func getProxyURL(r *http.Request) string {

backend/cmd/headlamp.go:634

• [nitpick] The error message has a trailing space. Removing it will produce cleaner output.

			http.Error(w, "no allowed proxy url match, request denied ", http.StatusBadRequest)

backend/cmd/headlamp.go:703

• The writeProxyResponse function reads and logs the response body but never writes the status, headers, or body back to the client. You should copy resp.Header to w.Header(), set the appropriate status code, and write respBody to w.

func writeProxyResponse(w http.ResponseWriter, resp *http.Response) {

[SinghaAnirban005]

/cc @illume

[skoeva]

looks like there's a merge conflict, a rebase should fix that

[SinghaAnirban005]

/cc @illume

[illume]

To reduce the risk of this change, and to help make verifying this is correct... it would be good to break this up into separate commits.

Additionally, if a bug is introduced we can more easily bisect the changes to see which part caused the issue.

[k8s-ci-robot]

PR needs rebase.

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[illume]

Closing in favour of

• backend: Refactor createHeadlampHandler to reduce complexity #3665