feat(datasource/dotnet-version): support custom registry URLs

[MLuc24]

Changes

DotnetVersionDatasource read its releases index from this.defaultRegistryUrls[0] instead of the registryUrl it was handed, so a configured registry was accepted by config and then ignored. It now uses the URL it is passed, and customRegistrySupport is true.

Two things came with that:

• The cache key gains the registry URL. It was config.packageName alone, which was correct while every lookup hit the same index, but once a registry can be configured two of them would share one entry and one registry's releases could be served for another. It is now ${registryUrl}:${packageName}, matching what node-version and rust-version already do. This invalidates existing entries once.
• The readme says what a custom registry means here. Unlike most datasources the default is a full file URL, not a host, so registryUrls is the URL of a mirrored releases-index.json. The channel files are then fetched from the URLs listed inside that index, so a mirror needs to point at its own copies — worth stating rather than leaving people to infer it.

The missing-registryUrl branch is unreachable in practice, since config fills it from defaultRegistryUrls; it carries the /* v8 ignore next */ comment the other datasources use for the same guard.

Context

Please select one of the following:

• This closes an existing Issue, Closes: datasource/dotnet-version: add customRegistrySupport #45028
• 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

AI assistance disclosure

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

• 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):

The code, the test and this description were written by Claude Opus 5 running in Claude Code, directed by @MLuc24. The shape follows the existing node-version and rust-version datasources rather than being invented: the guard idiom, the v8 ignore comment and the registry-scoped cache key are all copied from them. The new test was run against unpatched main first to confirm it fails there.

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Documentation (please check one with an [x])

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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

fetches the index from a configured registry mocks a mirror serving both the index and the channel file it points at, and asserts the releases come back. Because httpMock fails on any unmocked request, the test only passes if the mirror — and not the default host — was the one that got called.

On main, with only the test applied, it fails:

× fetches the index from a configured registry
AssertionError: expected undefined to be 'https://github.com/dotnet/sdk'
GET https://example.com/foo/bar/fail 404

With the change: Tests 10 passed (10).

Also run on the changed files: oxlint, biome check, prettier --check, and tsc --noEmit — all clean.

I did not run this against a real repository: it would need a self-hosted mirror of the .NET release metadata to exercise the path that actually changed.

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