Detect NoWarn NU1701 in SDK project discovery and warn during report

[brettfo]

Add HasNoWarnNU1701 property to ProjectDiscoveryResult that is set during SDK project discovery when the NoWarn property contains NU1701. The warning is reported once per project during ReportDiscovery using a fully normalized rooted path in sorted order.

The NU1701 warning is used to indicate that a project's target framework isn't compatible with the frameworks supported by the given package. An example might be a package that only explicitly supports net48 but a project that targets net10.0. A developer might do this if they have very specific knowledge about how a package is used and what may or may not work at runtime, but there is no granularity offered and dependabot has no way of knowing under what circumstances the differing target frameworks will matter or not. The likely result is that dependabot will fail to produce an update because during the compatibility checks it will find an "unsupported" package and abandon everything. With this approach we at least surface a warning so the user can easily see why no PR was generated.

I initially attempted to detect the bad package pairings before the update and not re-check those later, but that opens a huge can of worms around understanding exactly why the developer added NU1701 to NoWarn to begin with and we still have no way of knowing what other interactions might cause problems or not.

Eventually we can try to improve this behavior, but with this PR there is no functional change, just more reporting about why nothing happened.

[Copilot]

Pull request overview

This PR adds discovery-time detection of projects that suppress NuGet warning NU1701 via the MSBuild NoWarn property, and surfaces that information as a warning during discovery reporting. This improves diagnostics for cases where compatibility checks may lead Dependabot’s NuGet update flow to abandon updates.

Changes:

• Add HasNoWarnNU1701 to ProjectDiscoveryResult and set it during SDK project discovery by inspecting the evaluated NoWarn property.
• Emit a warning in ILogger.ReportDiscovery for each affected project, using a fully-normalized rooted project path and deterministic ordering.
• Add a unit test ensuring the warning is logged only for projects that set the flag and that output ordering is sorted.

Show a summary per file

File	Description
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/Utilities/ILogger.cs	Logs NU1701 suppression warnings per project during discovery reporting, with normalized + sorted paths.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/Discover/SdkProjectDiscovery.cs	Detects NU1701 in evaluated NoWarn and records it on the discovery result.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core/Discover/ProjectDiscoveryResult.cs	Adds HasNoWarnNU1701 flag to carry discovery metadata into reporting/JSON.
nuget/helpers/lib/NuGetUpdater/NuGetUpdater.Core.Test/Utilities/LoggerTests.cs	Verifies warning emission and ordering behavior for ReportDiscovery.

Copilot's findings

• Files reviewed: 4/4 changed files
• Comments generated: 0