test: cover lint command output and exit codes

[FanouZeng-TT]

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.

[damaz91]

Thanks @FanouZeng-TT !

Since run_lint also handles a couple other CLI flags and error states, we could optionally expand mod lint_command (either here or in a follow-up PR) to cover:

• Missing path: verifying that pointing lint to a non-existent path exits with code 2.
• --strict flag: verifying that a schema with warnings exits with code 1 when --strict is enabled.
• --quiet flag: verifying that progress output is suppressed.

[FanouZeng-TT]

Thanks for the follow-up suggestions. I’ll keep the current test-only change focused and cover the missing-path, strict-warning, and quiet-output cases together with the adjacent CLI behavior work, so they can share one coherent test matrix rather than expanding this PR piecemeal.