feat(nuget): allow disabling NuGet registries via hostRules configuration

[loganprice]

Changes

Avoid injecting the default nuget.org registry and other lookup registry URLs into the ephemeral nuget.config during artifact updates if the repository already has its own NuGet.config file. This resolves PackageSourceMapping errors in environments with strict mappings.

Additionally, respect enabled: false configured for NuGet feeds in hostRules (e.g. api.nuget.org), preventing disabled feeds from being queried or injected.

This path was chosen to leverage existing configuration options and to avoid introducing a breaking change.

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

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

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The public repository:

[RahulGautamSingh]

lgtm