Add Anthropic support for code generation

What does this MR do and why?

Similar to !132369 (merged) adds Anthropic support for code generation.

The changes are behind a code_generation_anthropic feature flag.

Rollout issue #426506 (closed)

Since there is no support yet on the AI Gateway, the MR only prepares the Anthropic prompts for code generation.

Related to #425890 (closed)

MR acceptance checklist

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