[dmt] require x-config-version when conversions exist

[diyliv]

Summary

• Lint rule (conversions.go): when openapi/conversions/ directory exists with valid conversion files but openapi/config-values.yaml has no x-config-version, dmt now reports an error instead of silently passing. Also checks that x-config-version matches the latest conversion version.
• Tester (tester.go): the conversion tester now reports an error when conversions exist but x-config-version is 0, instead of returning "not applicable".
• Tests: added unit tests for both the lint rule and tester covering all scenarios.

Context

When a module has conversion files but x-config-version is missing from config-values.yaml, the Deckhouse platform cannot generate the ModuleSettingsDefinition with schemas for all versions — the UI shows no configuration form for the module. See deckhouse/sds-replicated-volume#680 for a concrete example.

Example

Module with openapi/conversions/v2.yaml but openapi/config-values.yaml missing x-config-version:

# openapi/config-values.yaml — no x-config-version field
type: object
properties:
  dataNodes:
    type: object
# ...

# openapi/conversions/v2.yaml exists
version: 2
conversions:
  - del(.enableThinProvisioning)

Before: dmt lint passes silently, dmt test conversions skips the module.

After: dmt lint errors:

x-config-version is not set in config-values.yaml, but conversions exist

dmt test conversions errors with the same message.