Introduce TemplateRenderer for prompt templating

- Introduce new TemplateRenderer API providing the logic for rendering an input template.
- Update the PromptTemplate API to accept a TemplateRenderer object at construction time.
- Move ST logic to StTemplateRenderer implementation, used by default in PromptTemplate. Additionally, make start and end delimiter character configurable.

Relates to gh-2655

Signed-off-by: Thomas Vitale <[email protected]>

Author: ThomasVitale

spring-ai-client-chat/src/test/java/org/springframework/ai/prompt/PromptTemplateTest.java

@@ -118,7 +118,7 @@ public void testRenderWithList() {
 
 		PromptTemplate unfilledPromptTemplate = new PromptTemplate(templateString);
 		assertThatExceptionOfType(IllegalStateException.class).isThrownBy(unfilledPromptTemplate::render)
-			.withMessage("Not all template variables were replaced. Missing variable names are [items]");
+			.withMessage("Not all variables were replaced in the template. Missing variable names are: [items].");
 	}
 
 	@Test

spring-ai-client-chat/src/test/java/org/springframework/ai/prompt/PromptTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023-2024 the original author or authors.
+ * Copyright 2023-2025 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,7 +18,6 @@
 
 import java.util.HashMap;
 import java.util.Map;
-import java.util.Set;
 
 import org.assertj.core.api.Assertions;
 import org.junit.jupiter.api.Test;
@@ -45,7 +44,7 @@ void newApiPlaygroundTests() {
 		// Try to render with missing value for template variable, expect exception
 		Assertions.assertThatThrownBy(() -> pt.render(model))
 			.isInstanceOf(IllegalStateException.class)
-			.hasMessage("Not all template variables were replaced. Missing variable names are [lastName]");
+			.hasMessage("Not all variables were replaced in the template. Missing variable names are: [lastName].");
 
 		pt.add("lastName", "Park"); // TODO investigate partial
 		String promptString = pt.render(model);
@@ -93,44 +92,6 @@ void newApiPlaygroundTests() {
 
 	}
 
-	@Test
-	void testSingleInputVariable() {
-		String template = "This is a {foo} test";
-		PromptTemplate promptTemplate = new PromptTemplate(template);
-		Set<String> inputVariables = promptTemplate.getInputVariables();
-		assertThat(inputVariables).isNotEmpty();
-		assertThat(inputVariables).hasSize(1);
-		assertThat(inputVariables).contains("foo");
-	}
-
-	@Test
-	void testMultipleInputVariables() {
-		String template = "This {bar} is a {foo} test";
-		PromptTemplate promptTemplate = new PromptTemplate(template);
-		Set<String> inputVariables = promptTemplate.getInputVariables();
-		assertThat(inputVariables).isNotEmpty();
-		assertThat(inputVariables).hasSize(2);
-		assertThat(inputVariables).contains("foo", "bar");
-	}
-
-	@Test
-	void testMultipleInputVariablesWithRepeats() {
-		String template = "This {bar} is a {foo} test {foo}.";
-		PromptTemplate promptTemplate = new PromptTemplate(template);
-		Set<String> inputVariables = promptTemplate.getInputVariables();
-		assertThat(inputVariables).isNotEmpty();
-		assertThat(inputVariables).hasSize(2);
-		assertThat(inputVariables).contains("foo", "bar");
-	}
-
-	@Test
-	void testBadFormatOfTemplateString() {
-		String template = "This is a {foo test";
-		Assertions.assertThatThrownBy(() -> new PromptTemplate(template))
-			.isInstanceOf(IllegalArgumentException.class)
-			.hasMessage("The template string is not valid.");
-	}
-
 	@Test
 	public void testPromptCopy() {
 		String template = "Hello, {name}! Your age is {age}.";