fix: Handle markdown-wrapped JSON in LLM schema generation (#1663)

[YuriNachos]

Summary

• Strip markdown code block markers from LLM responses before JSON parsing
• Fixes JSONDecodeError when LLMs return JSON wrapped in ```json...```

Fixes

Fixes #1663

Details

Claude Sonnet and other LLMs sometimes return valid JSON wrapped in markdown
code blocks, even when JSON mode is enabled. This causes json.loads() to fail
with:

```
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```

The fix pre-processes the LLM response to remove common markdown patterns:

• ```json\n...\n```
• ```\n...\n```

Before attempting JSON parsing in JsonCssExtractionStrategy.generate_schema().

Test plan

• Code review confirms the fix handles common markdown wrapping patterns
• Preserves valid JSON responses without markdown
• Maintains backward compatibility with existing behavior

🤖 Generated with Claude Code