[Invoices]: Implement OioublHandler XML generation (OIOUBL — Denmark)

[nielsdrost7]

Context

Sub-story of #767. OioublHandler (OIOUBL — Denmark) currently throws RuntimeException('... not yet implemented ...') instead of generating real XML.

Recommended package

Likely num-num/ubl-invoice (already a dependency). OIOUBL is a Danish UBL 2.1 profile, not a distinct XML syntax — same reasoning as the EHF sub-story.

This needs a research spike first: confirm the library's field coverage satisfies the real OIOUBL specification before committing to it.

Scope

1. Research spike: verify num-num/ubl-invoice covers OIOUBL's mandatory fields
2. Implement OioublHandler::generateXml(), likely reusing most of PeppolBisHandler's object graph with OIOUBL-specific customization/profile IDs

Acceptance criteria

• Research spike findings documented
• OioublHandler::generateXml() returns real, well-formed OIOUBL XML
• Output passes PeppolXmlValidator
• Unit test asserts well-formedness and key fields, following the PeppolBisHandlerTest pattern
• FormatHandlersTest's unimplemented-formats test is updated to drop OIOUBL from the list

Sequencing note

Pair with #780 — same research spike question applies to both. See #767 for full package-research context.