Fix manifest import rejecting valid metadata with non-'none' language keys

[cubap]

Problem

The manifest import endpoint (POST /project/import?createFrom=URL) was rejecting valid IIIF manifests when metadata values used language tags other than "none".

Root Cause

validatePayload.js only accepted:

1. Plain string values: { value: "John Doe" }
2. Language maps with "none" key: { value: { "none": ["John Doe"] } }

Valid IIIF manifests can use any IETF language tag ("en", "fr", "la", etc.).

Changes

• utilities/validatePayload.js — Added validateLanguageMap() and validateMetadataField() helpers to accept any IETF language tag in metadata label and value fields. Refactored duplicated label/value validation into a single parameterized helper.
• test/local/utility.validation.test.js — Added 5 test cases covering non-"none" language tags, mixed formats, and invalid language maps.

Testing

All 14 local tests pass, including the 5 new metadata validation tests.

Related

• Fixes Manifest import rejects valid metadata with non-'none' language keys #530
• Related to TPEN-interfaces#567

[thehabes]

Made some minor changes during review and testing. Tested by importing various Manifests with different kinds of valid and invalid metadata properties. I metadata as expected.