Template variable handling in ChatPromptTemplate #29034

[SANTHOSH-SACHIN]

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Title: core: fix template variable handling in ChatPromptTemplate BaseMessage objects #29043

Description:
Fixes a bug in ChatPromptTemplate where template variables in BaseMessage objects (like SystemMessage) were not being processed. Previously, template variables in direct BaseMessage instances would be left unformatted, while variables in MessagePromptTemplate instances were processed correctly. This PR modifies the format_messages method to properly handle template variables in all message types.

Example of the bug:

template = ChatPromptTemplate.from_messages([
    SystemMessage("Answer in {language}"),  # Variable not processed
    HumanMessagePromptTemplate.from_template("Hello")  # Works fine
])

Before fix:

result = template.invoke({"language": "English"})
# Output: SystemMessage(content='Answer in {language}')  # Variable not replaced

After fix:

result = template.invoke({"language": "English"})
# Output: SystemMessage(content='Answer in English')  # Variable properly replaced

The fix adds template variable processing for BaseMessage objects while maintaining backward compatibility and existing functionality for other message types.

Key changes:

• Modified format_messages method to detect and process template variables in BaseMessage content
• Added template variable processing using PromptTemplate for BaseMessage instances
• Implemented proper error handling for missing variables or formatting failures
• Added comprehensive tests for template variable processing in all message types
• Maintained backward compatibility with existing usage patterns

Implementation details:

• Uses PromptTemplate for consistent variable formatting
• Only attempts formatting when template variables are detected
• Preserves original message if formatting fails
• Maintains all existing functionality for MessagePromptTemplate instances

@baskaryan, @efriis, @eyurtsev, @ccurme

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