support basic text output in tests

[pelikhan]

Introduce 'format' to PromptTest and refactored transforms logic.

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• 📝 Updated test documentation to clarify that GenAIScript now extracts the "text" field by default, with an option to use raw JSON output by specifying format: "json".
• ⚙️ Enhanced test configuration logic to support dual output formats ("text" and "json") with corresponding transform mappings for both test execution and assertions.
• 🔧 Modified type definitions to include an optional format field, enabling developers to explicitly choose the desired output format for tests.

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Pull Request Summary: Enhancing Prompt Testing

The push introduces several critical improvements for prompt-related testing:

• New Interfaces: PromptTest now supports arrays of custom assertions, enabling detailed test validation. The addition of ContentSafetyOptions allows specifying both content safety rules and format preferences.

• Format Specification: Users can now set output formats via the provided format option within the request context, defaulting to "text", aligning closely with public API expectations.

• Error and Value Handling: Enhanced error messages improve developer understanding of failures. Added conditional checks ensure that only relevant formatting is applied per JSON content type when json is requested in ContentSafetyOptions.

These changes collectively make testing more robust and maintainable by extending assertion capabilities, improving format handling, and refining error communication through the updated API.

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[github-actions]

The sentence structure could be improved for clarity. Consider rephrasing to avoid repetition and enhance readability.

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[github-actions]

LGTM with some notes:

• The transform variable in test.ts is defined but never used
• The format parameter in prompt_template.d.ts is documented but undefined usage
  This doesn't break anything but could be cleaned up for better maintainability.

Here are prrrogate fixes to remove unused variables:

--- a/packages/core/src/types/prompt_template.d.ts
+++ b/packages/core/src/types/prompt_template.d.ts
@@ -507,14 +507,10 @@
      * Additional deterministic assertions.
      */
     asserts?: PromptAssertion | PromptAssertion[]
 
-    /**
-     * Determines what kind of output is sent back to the test engine. Default is "text".
-     */
-    format?: "text" | "json"
-
+    /// Determines what kind of output is sent back to the test engine. Default is "text".
   }
 
 interface ContentSafetyOptions {

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