test: cover lint command output and exit codes - #53

Description

src/linter.rs has unit coverage for individual diagnostics, but the public
ucp-schema lint command had no CLI integration tests. Other commands in
tests/cli_test.rs cover their argument handling, exit codes, and output, while
lint's command boundary remained unpinned:

fn run_lint(path: &Path, format: &str, strict: bool, quiet: bool) -> Result<(), u8>

A regression in clap wiring, the error exit code, or JSON serialization could
therefore pass all linter unit tests.

Fix: add CLI contract tests for a valid schema (exit 0), a broken internal
reference (E003, exit 1), and parseable --format json output with the
expected aggregate fields.

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 test-only change.)

Screenshots / Logs (if applicable)

N/A — the new lint CLI tests pass 3/3; the full Rust suite passes 318/318. Full pre-commit, rustfmt, codespell, and git diff --check also pass.