feat: add cue_vet_test rule for validating data files against CUE schemas

[jaqx0r]

Summary

Adds a cue_vet_test Bazel test rule that runs cue vet to validate YAML/JSON data files against a cue_instance schema.

API

load("@rules_cue//cue:cue.bzl", "cue_vet_test")

cue_vet_test(
    name = "validate_config",
    instance = ":my_schema_instance",
    data = ["config.yaml", "overrides.yaml"],
    schema_expression = ["#Config"],
    concrete = True,  # default
)

Attributes

Attribute	Type	Default	Description
instance	label	required	cue_instance target providing the schema
data	label_list	required	YAML/JSON files to validate
schema_expression	string_list	[]	-d expressions selecting schema definitions
concrete	bool	True	Pass -c flag (require concrete values)

Implementation Notes

• New Bazel test rule (rule(test = True, ...)) — pass = exit 0, fail = non-zero + error in test log
• Generates a dedicated shell test runner via ctx.actions.write (does not reuse cue-run-from-runfiles, which assumes --outfile output production)
• Uses existing CUE toolchain (//tools/cue:toolchain_type)
• Instance path computed module-relative via paths.relativize (same as _make_instance_consuming_action)
• Runfiles resolved via rlocation using _runfile_path (existing helper, not duplicated)

Testing

• test/testdata/vet/ — new testdata directory with #Config schema, valid.yaml, passing cue_vet_test target

Closes #27