feat: add cross-database foreign key configuration

[amitdeshmukh]

Summary

• Adds colon-separated database prefix syntax to related_to config for cross-database FK references (e.g. ats:employees.id)
• Resolves FK targets against the correct database's schema metadata instead of only the local database
• Creates shadow nodes in the schema graph for cross-database targets, enabling the existing RelDatabaseJoin runtime path

Details

Previously, configuring related_to: "ats.employees.id" on a table in ats_orders would fail because GraphJin parsed it as schema.table.column and looked it up in the local database only. The new syntax ats:employees.id unambiguously separates the database name from the schema/table/column path and resolves the target column against the correct database's DBInfo.

Backward compatible — values without : parse identically to the existing behavior.

Test plan

• Unit tests for getFK parsing with all syntax variants (2-part, 3-part, cross-db 2-part, cross-db 3-part, invalid)
• Unit test for cross-database FK resolution via addForeignKeys
• Unit test for unknown database error handling
• Unit test for same-database FK regression (no FKeyDatabase set)
• Unit test for shadow node creation in schema graph with IsCrossDatabase() verification
• All existing core unit tests pass
• Postgres integration tests pass
• MSSQL integration tests pass

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