Add CODEOWNERS entries for AI integration sample directories (#501)

Author: xumaple

.github/CODEOWNERS

@@ -4,4 +4,7 @@
 # someone opens a pull request.
 * @temporalio/sdk
 /ai-sdk/           @temporalio/sdk @temporalio/ai-sdk
+/langsmith/        @temporalio/sdk @temporalio/ai-sdk
+/openai-agents/    @temporalio/sdk @temporalio/ai-sdk
+/strands-agents/   @temporalio/sdk @temporalio/ai-sdk
 /workflow-streams/ @temporalio/sdk @temporalio/ai-sdk