Fix @WithCodec checker to use correct codec type validation

Author: Mr3zee

compiler-plugin/compiler-plugin-k2/src/main/kotlin/kotlinx/rpc/codegen/checkers/FirWithCodecDeclarationChecker.kt

@@ -57,13 +57,14 @@ object FirWithCodecDeclarationChecker {
 
         val codecTargetClass = codecClassSymbol.findMessageCodecSuperType(context.session)
             .typeArguments.first().type
+            ?: error("Unexpected unresolved type argument for @WithCodec annotation")
 
-        if (codecTargetClass?.classId != declaration.symbol.classId) {
+        if (codecTargetClass.classId != declaration.symbol.classId) {
             reporter.reportOn(
                 source = withCodec.findArgumentByName(CODEC_ARGUMENT_NAME)?.source,
                 factory = FirGrpcDiagnostics.CODEC_TYPE_MISMATCH,
                 a = declaration.symbol.defaultType(),
-                b = kClassValue,
+                b = codecTargetClass,
                 context = context,
             )
         }

compiler-plugin/compiler-plugin-k2/src/main/templates/kotlinx/rpc/codegen/checkers/diagnostics/RpcKtDiagnosticsContainer.kt

@@ -32,6 +32,7 @@ fun registerDiagnosticRendererFactories() {
 //##csm specific=[2.0.0...2.2.10]
     RootDiagnosticRendererFactory.registerFactory(FirRpcDiagnostics.getRendererFactoryVs())
     RootDiagnosticRendererFactory.registerFactory(FirRpcStrictModeDiagnostics.getRendererFactoryVs())
+    RootDiagnosticRendererFactory.registerFactory(FirGrpcDiagnostics.getRendererFactoryVs())
 //##csm /specific
 //##csm /registerDiagnosticRendererFactories
 }