Expected Behavior

The RewriteQueryTransformer should allow custom prompt templates without requiring the {target} placeholder when the custom prompt doesn't use it.

// This should work with a custom prompt that doesn't need {target}
RewriteQueryTransformer.builder()
    .chatClientBuilder(chatClientBuilder)
    .promptTemplate(customPromptTemplate) // Custom prompt without {target}
    .build(); // Should not throw IllegalArgumentException

Current Behavior

RewriteQueryTransformer currently enforces that all prompt templates must include the {target} placeholder, even when using completely custom prompts that don't need this placeholder.

// This fails with IllegalArgumentException even though custom prompt doesn't use {target}
RewriteQueryTransformer.builder()
    .chatClientBuilder(chatClientBuilder)
    .promptTemplate(customPromptTemplate) // Custom prompt without {target}
    .build(); // Throws: "The following placeholders must be present in the prompt template: target"

Error Log:

java.lang.IllegalArgumentException: The following placeholders must be present in the prompt template: target
	at org.springframework.ai.rag.util.PromptAssert.templateHasRequiredPlaceholders(PromptAssert.java:55)
	at org.springframework.ai.rag.preretrieval.query.transformation.RewriteQueryTransformer.<init>(RewriteQueryTransformer.java:71)
	at org.springframework.ai.rag.preretrieval.query.transformation.RewriteQueryTransformer$Builder.build(RewriteQueryTransformer.java:128)

Context

How has this issue affected you?
I'm implementing a custom query transformation system that performs specialized processing beyond the default query rewriting. My custom prompt template doesn't need the {target} placeholder but the framework forces me to include it, causing runtime exceptions.

What are you trying to accomplish?
Create a flexible query transformation system that can handle custom business logic without being constrained by framework assumptions about required placeholders.

What other alternatives have you considered?

1. Using a different transformer class (but RewriteQueryTransformer is the most appropriate for this use case)
2. Implementing a custom transformer from scratch (overkill for this simple requirement)

Are you aware of any workarounds?
Yes, I can add a dummy {target} placeholder to my custom prompt template, but this is a hack that makes the code less clean and the prompt less readable. The placeholder validation should be conditional based on what's actually used in the custom prompt.

Example Use Case:

// Custom prompt for specialized query transformation
private val customPromptTemplate = PromptTemplate("""
    You are a query processor for a specialized analysis system.
    
    TASK: Process the user query to extract specific information.
    
    INPUT: {query}
    
    OUTPUT FORMAT (JSON):
    {
      "rewritten_query": "processed query",
      "custom_field": "custom value"
    }
    
    Query: {query}
    """)

This prompt doesn't need {target} but the framework currently requires it, causing the application to crash at runtime.