feat: add lint option to show all schema errors

[jmbenedetto]

Summary

• add an opt-in --all-errors lint mode that reports all JSON Schema validation errors instead of only the first one.
• keep the existing default fastjsonschema path unchanged, and use jsonschema.iter_errors(...) only for the all-errors path.
• add focused tests and a multi-error fixture, and expose the same behavior in the /lint API via all_errors=true.

Validation

• uv run --extra dev pytest tests/test_lint.py
• uv run --extra dev ruff check datacontract/cli.py datacontract/api.py datacontract/data_contract.py datacontract/lint/resolve.py datacontract/model/exceptions.py tests/test_lint.py

Notes

• This change was vibecoded in OpenCode using GPT-5.4.

[jschoedl]

Hi, thank you for your PR! Please run ruff format, add a Changelog entry, and update README. LGTM besides that.

[jmbenedetto]

Done — I ran ruff format, added a CHANGELOG entry, and updated the README for the new --all-errors lint mode.