flowey: add hint if dotnet restore fails

[benhillis]

This change adds some more actionable steps on how to resolve dotnet restore issues.

[Copilot]

Pull request overview

Adds a more actionable local-only hint when dotnet restore fails during Flowey’s NuGet package installation, aiming to reduce friction when restoring from authenticated feeds.

Changes:

• Wrap dotnet restore execution to emit an additional hint on failure for the Local backend.
• Return the original restore error after emitting the hint.

[Copilot]

This hint is shown for any dotnet restore failure on the Local backend, but the suggested actions (Azure Artifacts Credential Provider / az login / “see the repo’s getting started guide”) won’t apply in some cases (e.g., no Azure DevOps feeds were detected so get_feed_endpoints_json returned None, or az login failures would have already surfaced earlier). Consider gating this hint on whether the nuget.config actually contains Azure DevOps feeds (or whether feed_endpoints_json is Some), and either update Guide/src/ to include the referenced setup steps or change the message to point to a specific doc/location that actually contains these instructions.

Suggested change

	log::error!("HINT: NuGet restore failed. You may need to install \
	the Azure Artifacts Credential Provider and/or log in with \
	`az login`. See the repo's getting started guide for setup \
	instructions.");
	if feed_endpoints_json.is_some() {
	log::error!(
	"HINT: NuGet restore failed while using Azure DevOps feeds. \
	You may need to install the Azure Artifacts Credential \
	Provider and/or log in with `az login` to refresh your \
	credentials."
	);
	} else {
	log::error!(
	"HINT: NuGet restore failed. Check the restore output above \
	for details and ensure your NuGet feeds are accessible from \
	this environment."
	);
	}

[github-actions]

1 Petri tests failed (1 unstable)