No way to author a message-definition-sourced mapping — 74 of 327 real mappings use one, and DESCRIBE drops the source silently #263

A mapping can be sourced from a JSON structure, an XML schema, or a message
definition. MDL has a with clause for the first two
(mdl/grammar/domains/MDLDomainModel.g4:460) and none for the third, even though
the model field exists (model.ImportMapping.MessageDefinition) and is read by
show import mappings.

74 of 327 documents in the demo apps use one — the second-largest gap after the
array root (#248), and the two overlap heavily because message-definition mappings
are usually array-rooted.

Examples: AgentCore.Email_Import (AgentCore.MesDef_Email.Email),
AgentCore.EM_Workflows, GenAICommons.EM_Reference
(GenAICommons.MD_Reference.Reference), Email_Connector.EXM_EmailTemplateMapping,
MendixSSO.AppRolesResponse, ApplicationCore.Trace_Export.

It fails silently through DESCRIBE

describe import mapping MendixSSO.AppRolesResponse emits no with clause at
all, and the output parses:

create or modify import mapping MendixSSO.AppRolesResponse
  folder 'Internal (do not change)/AppRolesAPI/Configuration'
{
  create MendixSSO.AppRole {
    UUID = UUID,
    DisplayName = DisplayName
  }
};

Re-executing that produces a source-less mapping bound to nothing. 70 of the 74
land in the silent class of #260.

Suggested

with message definition Module.Doc.MessageName

The reference is three-part (document + message) — that is how it is stored, e.g.
Email_Connector.MD_EmailTemplate.EmailTemplate_Msg_Definition — and Studio Pro's
PED schema declares it as
messageDefinition?: Reference<'MessageDefinitions$MessageDefinition','qualified-name'>.

As with with xml schema, the definition must already exist in the project;
create message definition is a separate concern and marketplace modules ship the
definitions anyway. The reference must be resolved, not written verbatim — see
#259.

Measured on 70a12365. Context: docs/11-proposals/PROPOSAL_mapping_coverage.md §7.3.