Allow markdown or custom formatting for code blocks

[aymeric-roucher]

This follows up on #1442 and supersedes and closes #1491:

• In Change code tags to xml #1442, I changed code block fencing blocks from markdown to XML to align with general usage of XML tags in chat templates
• While this does not negatively impact most models, it still degraded performance for Gemma-3-12B-IT, and maybe other models, we hypothese that this is due to presence of markdown-formatted blocks in training corpora. Thank you @peabody124 for raising the issue!
• This PR introduces the possibility to customize code block tags
  • It also adds a check for the presence of markdown blocks as a fallback in any case.
  • It also adapts the system prompt to whichever code block format is used. In particular, in the system prompts, it wraps the tool descriptions within these code tags, so as to further nudge the model towards using the proper tags.
    cc @albertvillanova

[albertvillanova]

Thanks! Good fix! Also, it was nice to remove the remaining <end_code> tags...

[albertvillanova]

Why you renamed it to initial_pattern instead of original pattern?

[aymeric-roucher]

It's a residue of implementation change, reverting this

[peabody124]

I can confirm using this branch with code_block_tags="markdown" reproduces the benefits of #1491 . Closing my PR.