chore: Add Jspecify NullCheck in tck module #336 (#340)

…y @nullable annotation to AgentCardProducer, getEnvOrDefault Method and
AgentExecutorProducer execute Method. Fixes #336

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Fixes #336  🦕

Author: ethanthopkins

tck/src/main/java/io/a2a/tck/server/AgentExecutorProducer.java

@@ -11,7 +11,6 @@
 import io.a2a.spec.JSONRPCError;
 import io.a2a.spec.Task;
 import io.a2a.spec.TaskNotCancelableError;
-import io.a2a.spec.TaskNotFoundError;
 import io.a2a.spec.TaskState;
 import io.a2a.spec.TaskStatus;
 import io.a2a.spec.TaskStatusUpdateEvent;

tck/src/main/java/io/a2a/tck/server/package-info.java

@@ -0,0 +1,10 @@
+@NullMarked
+package io.a2a.tck.server;
+
+import org.jspecify.annotations.NullMarked;
+
+//The following had @Nullable annotation applied from JSpecify
+//AgentCardProducer.java getEnvOrDefault method, 
+//AgentExecutorProducer.java execute method
+//
+