build: add FluentValidation host guard to prevent type-identity issues

[RicherTunes]

Summary

Adds build-time validation that FluentValidation.dll exists in the Lidarr host assemblies path. This prevents accidental builds where Directory.Packages.props pulls FV 11.x from NuGet instead of using the host's FluentValidation.

Problem (Contributor Footgun)

The current csproj has a conditional PackageReference:

<PackageReference Include="FluentValidation" 
                  Condition="!Exists('$(LidarrPath)\FluentValidation.dll')" />

If someone builds without proper host assemblies, NuGet silently provides FluentValidation 11.x. The build succeeds but the plugin fails at runtime with:

Method 'Test' in type 'Lidarr.Plugin.Brainarr.BrainarrImportList' does not have an implementation.

Solution

New MSBuild target ValidateFluentValidationFromHost:

• Runs before Build (when LidarrPath is set)
• Fails fast if $(LidarrPath)\FluentValidation.dll doesn't exist
• Clear error message with:
  • Why host FV is required
  • How to fix it
  • Link to PR fix(packaging): don't ship FluentValidation to avoid ImportList Test() type-identity mismatch #346 for context

Build Error Example

error : CRITICAL: FluentValidation.dll not found in ..\ext\Lidarr-docker\_output\net8.0. 
        Brainarr MUST use host's FluentValidation to avoid type-identity mismatch 
        (override signature for Test method). Extract Lidarr assemblies with 
        './build.ps1 -ExtractAssemblies' or use a complete Lidarr installation.

Related

• Closes footgun identified in fix(packaging): don't ship FluentValidation to avoid ImportList Test() type-identity mismatch #346 review
• Part of ecosystem packaging policy enforcement

🤖 Generated with Claude Code

[github-actions]

⚠️ Deprecation Warning: The deny-licenses option is deprecated for possible removal in the next major release. For more information, see issue 997.

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[RicherTunes]

Stale — superseded by packaging-gates adoption (#397) and ecosystem standardization.