Command plugin should skip irrelevant targets when run without --target argument

[simonjbeaumont]

Motivation

In #795 we adjusted the logic of the command plugin because errors were not propagating properly. As a result, we regressed behaviour when the command plugin was run without any --target argument. In this mode, the plugin runs on all targets. When it detects a misconfigured target it errors, as it should. However, the classification of misconfigured target included targets that were not intended for code generation at all, resulting in errors running the command plugin on packages with others targets.

Modifications

• Add command plugin generation step to integration test
• If the command plugin is running without a --target argument and cannot find any of the required files, skip that target.

Result

• Manual code generation with the command plugin fixed for packages with other targets.
• Fixes Manually generating code fails if package includes additional targets #797.

Test Plan

This PR includes two commits. The first adds a step to the integration test, which runs the command plugin on the integration test package. This will fail in CI. Then the second commit will provide the fix, which should pass in CI. The commits will then be squash merged.

[simonjbeaumont]

Good... the canary commit shows the failure:

error: Issues with required files:
- No config file found in the target named 'MockTransportClient'. Add a file called 'openapi-generator-config.yaml' or 'openapi-generator-config.yml' to the target's source directory. See documentation for details.
- No OpenAPI document found in the target named 'MockTransportClient'. Add a file called 'openapi.yaml', 'openapi.yml' or 'openapi.json' (can also be a symlink) to the target's source directory. See documentation for details..

— source: https://github.com/apple/swift-openapi-generator/actions/runs/16471435605/job/46561409290?pr=798#step:4:752

But... annoyingly we get badly ordered output since there's a mix of stderr and stdout logging from the plugin and tool. A well behaved tool should log to stderr and reserve stdout for actual output that could e.g. be piped to other processes.

I'll add one more commit to fix that before adding the commit for the actual fix.

[simonjbeaumont]

OK, great. This is now failing as we expect:

...
- ✅ OpenAPI code generation for target 'Server' successfully completed.
Considering target 'MockTransportClient':
- Trying OpenAPI code generation.
- OpenAPI code generation failed with error.

— source: https://github.com/apple/swift-openapi-generator/actions/runs/16471883278/job/46562960013?pr=798#step:4:818

I'll add the fix.

[simonjbeaumont]

And, now things are working as expected:

...
- ✅ OpenAPI code generation for target 'Server' successfully completed.
Considering target 'MockTransportClient':
- Trying OpenAPI code generation.
- Skipping because target isn't configured for OpenAPI code generation.
Considering target 'MockTransportServer':
- Trying OpenAPI code generation.
- Skipping because target isn't configured for OpenAPI code generation.
** ✅ Successfully built integration test package.

— source: https://github.com/apple/swift-openapi-generator/actions/runs/16472015174/job/46563435636?pr=798#step:4:804

[czechboy0]

Approving, feel free to decide what to do with the two comments