chore: enable forcetypeassert linter, fix issues (#38804)

Enable [`forcetypeassert`](https://github.com/gostaticanalysis/forcetypeassert)
linter to prevent unchecked type assertions. ~650 issues fixed, most
fixes were clean, some use `setting.PanicInDevOrTesting`.

The only behaviour changes are where code would previously send a 500 error
or panic, a 4xx error is now emitted.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/packages/rubygems/rubygems.go

@@ -75,7 +75,7 @@ func enumeratePackages(ctx *context.Context, filename string, pvs []*packages_mo
 				Name:  "Gem::Version",
 				Value: []string{p.Version.Version},
 			},
-			p.Metadata.(*rubygems_module.Metadata).Platform,
+			packages_model.DescriptorMetadata[*rubygems_module.Metadata](p).Platform,
 		})
 	}
 
@@ -126,7 +126,7 @@ func ServePackageSpecification(ctx *context.Context) {
 	zw := zlib.NewWriter(ctx.Resp)
 	defer zw.Close()
 
-	metadata := pd.Metadata.(*rubygems_module.Metadata)
+	metadata := packages_model.DescriptorMetadata[*rubygems_module.Metadata](pd)
 
 	// create a Ruby Gem::Specification object
 	spec := &rubygems_module.RubyUserDef{
@@ -405,7 +405,7 @@ func makePackageVersionDependency(ctx *context.Context, version *packages_model.
 		return "", err
 	}
 
-	metadata := pd.Metadata.(*rubygems_module.Metadata)
+	metadata := packages_model.DescriptorMetadata[*rubygems_module.Metadata](pd)
 	fullFilename := makeGemFullFileName(pd.Package.Name, version.Version, metadata.Platform)
 	file, err := packages_model.GetFileForVersionByName(ctx, version.ID, fullFilename, "")
 	if err != nil {