Mapping tables with composite keys

[csgalan]

This improve the handling of identification of mapping tables when include composite keys

[sjh37]

Thanks @cgalan12
This is causing a few issues with one of the large and nasty test databases I have. Tables with many fields used as primary keys seem to be missing. I will investigate further...

[sjh37]

I understand what you are trying to do, but will have to leave this here for now as it's not working for all databases. I will return to this once EF 7 (Core) is complete.

[sjh37]

This PR does the following:

Fixes mapping table detection for composite keys (EF6)

The old code assumed a mapping/junction table always has exactly 2 columns and 2 FK columns. This broke when the joined tables have composite primary keys (e.g. a many-to-many between two tables each with a 2-column PK would create a 4-column mapping table).

Changes:

1. IdentifyMappingTable — removes the hardcoded nonReadOnlyColumns.Count != 2 check, replacing it with "all non-read-only columns must be primary key columns" (works for any number of composite key columns).
2. FK distinctness check — switches from FkColumn.Distinct().Count() != 2 to ConstraintName.Distinct().Count() != 2, correctly identifying two distinct FK constraints regardless of column count.
3. right FK selection — changed from foreignKeys[1] (index-based, fragile) to foreignKeys.Find(f => f.ConstraintName != left.ConstraintName) (correct for multi-column FKs).
4. AddMappingConfiguration — new GetMapKeyName helper collects all columns for a given constraint and joins them as "col1", "col2" for MapLeftKey/MapRightKey.

It's not merge-ready as-is — it edits the old EF.Reverse.POCO.Core.ttinclude (v2 file), not the current Generator/ source files. The logic fix itself looks reasonable and addresses a real gap.