Refactor system message composition for better code proximity & allow user to enforce empty system message as required by Deepseek R1 series

[ferenci84]

Description

1. Moved systemMessage composition to a single place - makes it easy to make changes discussed in issue Proposed configuration option for better control over hard-coded system messages #3786
2. When user defined systemMessage is an explicit empty string, no system message will be created - this resolves Proposed configuration option for better control over hard-coded system messages #3786 (comment)

Note: #4507 is built on top of this one, that contains a more complete, yet experimental solution for the issue #3786

Checklist

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

Testing instructions

If you set empty systemMessage in the model config or in the top-level config, no systemMessage should be used (model config takes precedence even if it's an empty string)

[netlify]

✅ Deploy Preview for continuedev ready!

Name	Link
🔨 Latest commit	11f3351
🔍 Latest deploy log	https://app.netlify.com/sites/continuedev/deploys/67c9350b73741d000836fdcd
😎 Deploy Preview	https://deploy-preview-4506--continuedev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[ferenci84]

Other PR submitted on top of this one: #4507

[LLukas22]

Would be very nice to have this as an option 👍