Add structured output: JSON schema for ParseResult

[kiyeonjeon21]

What

Export the ParseResult Pydantic model as JSON Schema so other tools can validate parsemux output.

How

Add to CLI:
```bash
parsemux schema --output-schema # prints ParseResult JSON Schema
```

Implementation

```python
from parsemux.core.models import ParseResult
print(ParseResult.model_json_schema())
```

Files

• `src/parsemux/cli/main.py` — add to schema command

[abhijeetnardele24-hash]

I’m working on this and will open a PR for the ParseResult JSON schema output.