Ensure complete mappings are generated from old taxonomy versions to l…

[katbailey]

Problem

When generating dist mapping files (e.g., shopify_2025-09_to_shopify_2026-02.json), the chain resolution only resolves the outputs of existing rules forward through newer versions. It does not propagate rules from intermediate versions backward to older versions.

This means if a category ID existed in an older version but was only remapped in a later version, the older version's mapping file would have no entry for it — even though that ID is now invalid in the latest version. The mapping files claim to map old version → latest but are incomplete.

Approach

Modified resolve_to_shopify_mappings in IntegrationVersion to propagate rules backward after resolving existing rules. Since resolve_to_shopify_mappings_chain processes newest-to-oldest, by the time we process an older version, all newer versions' rules already have their outputs resolved to the latest taxonomy Category objects.

After the existing resolution loop, we now iterate through all newer versions' to_shopify_mappings and for each rule:

1. Check if the input category ID exists in the current (older) version's full_names_by_id
2. Check if the current version doesn't already have a rule for that input ID
3. If both conditions are true, create a propagated MappingRule mapping from the older version's category entry to the newer rule's already-resolved output

Existing rules always take precedence — propagation only fills gaps.

Test plan

• Added unit test: rule from newer version propagates to older version when category exists in full_names_by_id
• Added unit test: existing rules are not overwritten by propagated rules
• Added unit test: rules are not propagated for categories absent from full_names_by_id
• Updated load_all_from_source chain test to verify propagated rule count
• Added aa-2 to 2020-01 fixture's full_names.yml to cover the propagation scenario
• Run generate_dist and verify mapping file completeness