fix: reject unsupported lint output formats

[FanouZeng-TT]

Description

ucp-schema lint --help documents the output format as text or json, but
run_lint only checked for format == "json" and silently treated any other
value as text:

ucp-schema lint schema.json --format yaml
# exit 0, text output

That hides typos in scripts that expect machine-readable output.

Fix: reject unsupported lint output formats at the command boundary with
exit code 2 and a clear error message, while keeping text and json behavior
unchanged.

Category (Required)

• Core Protocol: ...
• Governance/Contributing: ...
• Capability: ...
• Documentation: ...
• Infrastructure: ...
• Maintenance: ...
• SDK: ...
• Samples / Conformance: ...
• UCP Schema: Changes to the ucp-schema tool. (Requires Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide ...
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk. (Not applicable: ucp-schema CLI fix.)

Screenshots / Logs (if applicable)

N/A — cargo test --test cli_test lint_command passed 1/1 for the new test, the full Rust suite passed 316/316, and full pre-commit plus git diff --check passed.