feat: add SQLModel support for Message Implementation Guides (MIG)

Add SQL model classes for MIGs mirroring the existing AHB SQLModel
implementation patterns:

- MigCode, MigDataElement, MigDataElementGroup, MigSegment,
  MigSegmentGroup, MigSegmentGroupLink, MessageImplementationGuide
- from_model() and to_model() conversion methods
- Position fields for stable list ordering
- Self-referential SegmentGroup relationship via link table
- SQL-only fields: gueltig_von, gueltig_bis, edifact_format_version

Includes roundtrip tests verifying XML -> Pydantic -> SQL -> Pydantic
equality for all example MIG files.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: hf-kklein

src/fundamend/sqlmodels/__init__.py

@@ -28,6 +28,15 @@
     Segment,
     SegmentGroup,
 )
+from .messageimplementationguide import (
+    MessageImplementationGuide,
+    MigCode,
+    MigDataElement,
+    MigDataElementGroup,
+    MigSegment,
+    MigSegmentGroup,
+    MigSegmentGroupLink,
+)
 
 __all__ = [
     "create_ahb_view",
@@ -46,4 +55,11 @@
     "Anwendungsfall",
     "Bedingung",
     "Anwendungshandbuch",
+    "MessageImplementationGuide",
+    "MigCode",
+    "MigDataElement",
+    "MigDataElementGroup",
+    "MigSegment",
+    "MigSegmentGroup",
+    "MigSegmentGroupLink",
 ]