fix(motoko): enforce useDfx/useIcp mutual exclusivity

Throw IllegalArgumentException in processOpts() when both flags are
set. Update CLI option descriptions to document the constraint.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ggreif

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/MotokoClientCodegen.java

@@ -147,8 +147,8 @@ public MotokoClientCodegen() {
         typeMapping.put("AnyType", "Text");
 
         cliOptions.add(CliOption.newString(PROJECT_NAME, "Project name for generated code"));
-        cliOptions.add(CliOption.newBoolean(USE_DFX, "Generate code for dfx with ic:aaaaa-aa imports", useDfx));
-        cliOptions.add(CliOption.newBoolean(USE_ICP, "Generate icp.yaml for deployment with icp-cli (canary; replaces dfx)", useIcp));
+        cliOptions.add(CliOption.newBoolean(USE_DFX, "Use ic:aaaaa-aa imports (dfx toolchain). Mutually exclusive with useIcp.", useDfx));
+        cliOptions.add(CliOption.newBoolean(USE_ICP, "Use ic:aaaaa-aa imports and generate icp.yaml (icp-cli toolchain; replaces dfx). Mutually exclusive with useDfx.", useIcp));
 
         // Enable inline enum resolution to create model files for inline enum parameters
         // This ensures type-safe enum variants instead of raw Text types
@@ -171,6 +171,9 @@ public void processOpts() {
         if (additionalProperties.containsKey(USE_ICP)) {
             setUseIcp(convertPropertyToBooleanAndWriteBack(USE_ICP));
         }
+        if (useDfx && useIcp) {
+            throw new IllegalArgumentException("useDfx and useIcp are mutually exclusive — pick one toolchain");
+        }
         additionalProperties.put(USE_ICP, useIcp);
         if (useIcp) {
             supportingFiles.add(new SupportingFile("icp.yaml.mustache", "", "icp.yaml"));