Allow setting BT-23 (BusinessProcessSpecifiedDocumentContextParameter/ID) for CII exports (Factur-X / EN16931 / EXTENDED)

[meparis]

Describe the problem
When generating CrossIndustryInvoice (CII) XML with ZUGFeRD2PullProvider, the element:

/rsm:CrossIndustryInvoice/rsm:ExchangedDocumentContext/ram:BusinessProcessSpecifiedDocumentContextParameter/ram:ID (EN16931 BT-23)

cannot be set by the user.

Currently Mustang only writes BusinessProcessSpecifiedDocumentContextParameter for the XRechnung profile (with a fixed/implicit value), and omits it for other profiles such as Factur-X EXTENDED / EN16931.
This makes it impossible to satisfy ecosystems/validators where BT-23 is required or expected.

Expected behavior
Users should be able to set a custom BT-23 value (string / URN) when generating CII XML, independently of the selected profile.

Actual behavior

In profile XRechnung: BT-23 is generated with a fixed value (hardcoded), but cannot be customized.

In profile EXTENDED/EN16931/Factur-X: BT-23 is not generated at all.

Why this matters / use cases

Some customers require a local business process code (e.g. “B1”, “B8”) for routing/workflow, and currently we must post-process the XML output (fragile).

Proposed solution (API)
Option A (preferred): add an optional field to Invoice:

Invoice#setBusinessProcessId(String id)

Invoice#getBusinessProcessId()

Then in ZUGFeRD2PullProvider:

if invoice.getBusinessProcessId() is not null/blank → always write ram:BusinessProcessSpecifiedDocumentContextParameterram:ID...</ram:ID></ram:BusinessProcessSpecifiedDocumentContextParameter>

else keep current behavior (XRechnung default value or omitted for other profiles)

Option B: add a setter on the provider:

ZUGFeRD2PullProvider#setBusinessProcessId(String id)

Backward compatibility

No breaking changes: default output remains unchanged unless the new property is set.

For XRechnung: keep the existing default value if user does not set anything; allow overriding when explicitly provided.

Minimal patch hint
BusinessProcessSpecifiedDocumentContextParameter is written in ZUGFeRD2PullProvider only under XRechnung conditions; the patch would move this logic to:

“if custom id present → write it; else if profile==XRechnung → write current default; else omit”.

Additional context

BT-24 (GuidelineSpecifiedDocumentContextParameter/ID) is already correctly written based on profile. The missing part is making BT-23 configurable.