feat(flux): map Helm sourceRef names via registryAliases

[tonur]

Changes

• Extend the Flux manager’s Helm resolution to fall back to registryAliases keyed by HelmRelease.spec.chart.spec.sourceRef.name/HelmChart.spec.sourceRef.name when no matching HelmRepository resource is found in scanned manifests.

• Preserve existing behavior when a matching HelmRepository exists (including OCI handling) and keep skipReason: 'unknown-registry' when neither a repository nor an alias is available.

• Add unit tests to cover the new alias-based resolution for both HelmRelease and HelmChart.

• Update the Flux manager documentation to describe how to use flux.registryAliases to statically map sourceRef.name values to registry URLs.

Context

Please select one of the following:

• This closes an existing Issue, Closes: #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation - Discussion Idea: Allow static mapping of Flux HelmRelease sourceRef names to URLs (for external/global HelmRepositories) #40007

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

I used Github Copilot to work on this feature.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository
  ran here: https://github.com/tonur/renovate-test-flux-static-registry-aliases/pull/2/changes

[tonur]

I have tested that this works for my private repository and a public repository created specifically for this purpose here: https://github.com/tonur/renovate-test-flux-static-registry-aliases/pull/2/changes

[viceice]

use a package rule in docs and add tests for full coverage