feat(PL-7072): allow specifying helm user for login

[davidmdm]

This PR allows us to configure the helm user we want to use during helm registry login.

This allows us to use different sources of authentication instead of the default _json_key such as oauth2accesstoken.

[coderabbitai]

📝 Walkthrough

Walkthrough

The operator and Helm chart now support a configurable Helm registry username. Configuration flows through HELM_USER and chart values into helm registry login, with rendered Helm fixtures and test setup behavior updated.

Changes

Helm registry authentication

Layer / File(s)	Summary
Helm username configuration contract chart/values.yaml, chart/tests/helm/values.yaml, cmd/operator/conf.go	Adds Helm username values and maps HELM_USER to HelmLogin.User, defaulting to _json_key.
Helm login parameter flow cmd/operator/main.go	Passes HelmLogin to authentication and uses its username, registry, and credentials for the Helm login command.
Rendered chart and test setup chart/templates/deployment.yaml, chart/tests/helm/expected.yaml, cmd/operator/main_test.go	Renders HELM_USER, updates expected chart resources, and adds SETUP_ONLY test-process handling.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly matches the main change: adding configurable Helm user support for registry login.

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/PL-7072/support-oauth2-registry-auth

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[nwaller-nesto]

seems reasonable