fix(imports): preserve link_hub_source_mapping for self-referencing links on JSON round trip

[tkirschke]

Context

Follow-up to #174. That PR made link_source_mapping survive JSON import (dependent-child-key / payload columns were being dropped). This PR closes the remaining gap on the other link mapping table, link_hub_source_mapping.

Note: the change in this PR was briefly pushed directly to main by mistake (bypassing review) and has been reverted on main (commit reverting c3fcfa9). This PR re-introduces it properly for review.

Problem

For a link that references the same hub more than once (e.g. customer_duplicate_customer_l, which points at CUSTOMER_H twice with aliases HK_CUSTOMER_H and HK_CUSTOMER_DUPLICATE_H), the JSON export could not say which source column feeds which reference. LinkColumnMapping carried only (link_column_name, link_column_type, source_column_name) — both business-key mappings serialized with the identical link_column_name, so the importer collapsed them onto the first reference with the wrong source column.

Verified against the bundled TPCH model: link_hub_source_mapping round-tripped 14 → 13 with one wrong row.

Fix

Add an optional target_foreign_hashkey to LinkColumnMapping — the hub hashkey alias (else hub hashkey name), which uniquely identifies a hub reference:

• models.py — new optional field (backward compatible; defaults to None).
• builder.py — populate it for business-key mappings from hub_ref.hub_hashkey_alias_in_link or hub.hub_hashkey_name.
• json_parser.py — resolve the hub reference by target_foreign_hashkey; fall back to the previous column-name match when the field is absent, so legacy exports still import.

Verification

• Full TPCH round trip (DB → JSON export → JSON import): both link_hub_source_mapping (14 → 14) and link_source_mapping (14 → 14) now come back with zero diff, including the same-hub-twice link, composite business keys, and a renamed foreign hashkey.
• New tests: whole-model round-trip equality (asserts both mapping tables as sets) + a legacy-export fallback test (strips target_foreign_hashkey).
• Full suite: 100 passed, 11 skipped. ruff clean.

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