Smart Apply: Use system prompt to encourage code blocks being created (…

…#5290)

## Description

This PR improves the smart apply prompt tweak so that:
- We use the system prompt when available
- We use the same desired Markdown syntax when _we_ include codebase
context, which helps steer the LLM onto the right path.

I have ran an eval on this here:
sourcegraph/cody-leaderboard#16


## Test plan

- [x] Tested creating code blocks on all major models

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Co-authored-by: Beatrix <[email protected]>