Spring AI StTemplateRenderer delimiter token allows multiple characters

Please do a quick search on GitHub issues first, the feature you are about to request might have already been requested.

Expected Behavior

StTemplateRenderer.builder().startDelimiterToken(“{{”).endDelimiterToken("}}").build()

Current Behavior

StTemplateRenderer.builder().startDelimiterToken('{').endDelimiterToken('}').build()

Context

Single Char is easy conflict with normal content in prompt.

Comment From: zjxzjw

Submitted PR, please refer to review # 3801