replaceSystemMessage model config option

[ferenci84]

Description

#3786

A model config option, named "replaceSystemMessage" (self-descriptive), default false.

Behavior:
When true, the model's systemMessage property, or the config's root systemMessage property will replace the default hard-coded system message (Something like "When generating new code:\n\n1. Always produce a single code block.\n2. Never separate the code into multiple code blocks.\n3. Only include the code that is being added.\n4. Replace existing code with a "lazy" comment like this ...." and also "When using tools, follow the following guidelines:\n- Avoid calling tools unless they are absolutely necessary...")

Setting false or not setting at all, will keep the current behavior.

Minor change:
The existing behavior, that the model config's systemMessage has higher precedence than the root config's systemMesage, was not documented, I added that to make the information in the documentation more obvious about the new config.

Checklist

• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screenshots

[ For visual changes, include screenshots. ]

Testing instructions

Testing replaceSystemMessage = true

1. Create a systemMessage in the default Claude model config.
2. Add replaceSystemMessage true in the model config.
3. Send any message with the model
4. Check the Continue LLM Prompt/Completion on the Output tab.
5. The system message in the sent request should now be the just the one given in the model config.
6. Remove the config and send the message again.
7. The system message should be appended to the default one.

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