Ensure unique functions in FunctionCallingOptionsBuilder

- Fix issues caused by cf75640
- Modify withFunction() to maintain unique function entries using HashSet

Author: tzolov

spring-ai-core/src/main/java/org/springframework/ai/model/function/FunctionCallingOptionsBuilder.java

@@ -61,7 +61,9 @@ public FunctionCallingOptionsBuilder withFunctions(Set<String> functions) {
 
 	public FunctionCallingOptionsBuilder withFunction(String function) {
 		Assert.notNull(function, "Function must not be null");
-		this.options.getFunctions().add(function);
+		var set = new HashSet<>(this.options.getFunctions());
+		set.add(function);
+		this.options.setFunctions(set);
 		return this;
 	}
 
@@ -131,7 +133,7 @@ public PortableFunctionCallingOptions build() {
 		return this.options;
 	}
 
-	public static class PortableFunctionCallingOptions implements FunctionCallingOptions, ChatOptions {
+	public static class PortableFunctionCallingOptions implements FunctionCallingOptions {
 
 		private List<FunctionCallback> functionCallbacks = new ArrayList<>();