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

[jaqx0r]

Summary

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

Motivation

cue vet is the standard way to validate data files (YAML, JSON) against CUE schemas (docs). There's currently no rules_cue support for this — users can only export or def, but cannot express "this YAML must conform to this CUE schema" as a Bazel test.

Proposed API

load("@rules_cue//cue:defs.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)

Behavior

• Pass: cue vet exits 0 (silent)
• Fail: cue vet exits non-zero, error output in test log

Implementation Notes

• New Bazel test rule (rule(test = True, ...))
• Consumes CUEInstanceInfo provider from instance attr
• Generates a shell test runner via ctx.actions.write (does not reuse cue-run-from-runfiles — that wrapper assumes output file production)
• Uses existing CUE toolchain (//tools/cue:toolchain_type)

Out of Scope

• Standalone file validation (no cue_instance) — could be added later
• cue vet on CUE-only packages (-c=false ./... pattern)