fix(CodeBlock): handle ReactNode children with angle-bracket placeholders

[mneedham]

Summary

• Changed children prop type from string to ReactNode in CodeBlock.types.ts
• Added nodeToString() helper that recursively converts ReactNode to a plain string, reconstructing <tag> syntax for React element nodes
• All internal uses of children as a string now go through nodeToString(children)

Problem

When code content contains <placeholder> style text (e.g. shell scripts with <project_id>, <email>), JSX parses those as React elements rather than raw text. SyntaxHighlighter then receives objects instead of a string and renders [object Object] throughout the block.

Test plan

• Pass a code block with <placeholder> style content and verify it renders literally
• Verify $VARIABLE style content still renders as-is
• Verify copy button copies the correct plain-text string
• Verify existing SQL/bash/JSON syntax highlighting still works

🤖 Generated with Claude Code

[changeset-bot]

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Latest commit: 636031e

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[punkbit]

@mneedham thanks for your contribution, mind add the missing changeset? Check https://github.com/clickHouse/click-ui?tab=readme-ov-file#add-a-new-changeset