fix(api-catalog): resolve discriminator.mapping refs to #/$defs pointers (DEVX-548)

[smukherjee-godaddy]

Problem

gddy api describe returned broken, unresolvable schema references for polymorphic commerce types (Amount, Fee, FundingSource, Channel, PricingModel, CallbackAuthentication).

The API catalog resolves $ref objects into local #/$defs/... entries, but OpenAPI discriminator.mapping carries schema references as plain strings, not $ref objects — so the dereferencer never rewrote them. The generated catalog kept external file paths like:

"discriminator": {
  "mapping": {
    "DEFAULT": "./BasicChannel.yaml",
    "APPLE_PAY": "../payment-token/ApplePayPaymentToken.yaml"
  }
}

A developer inspecting a commerce schema via api describe got a dead file path instead of a resolvable pointer. 52 such leaks existed across channels, payment-requests, payments, subscriptions, and transactions.

Fix

• Generator (rust/tools/generate-api-catalog): added rewrite_discriminator_mappings, run after ref-lifting in load_and_dereference. It rewrites each external mapping value to the matching #/$defs/<key> using the same key derivation as $ref lifting — and only when that def exists, so a mapping is never pointed at a missing schema.
• Build guard: extended the unresolved-reference check to treat external discriminator.mapping values as build-failing, exactly like an external $ref. Future spec drift now fails generation instead of silently shipping a broken pointer.
• Catalog: rewrote 52 mappings across the 5 affected domains. (Full regeneration requires source-spec repo access unavailable in this environment, so the committed catalog was rewritten with the identical, verified transform; the generator change keeps it correct on the next regen.)
• Tests: added mapping-rewrite and drift-guard unit tests.

Impact

• gddy api describe now returns fully-resolvable schemas for all polymorphic commerce types.
• The catalog contains zero unresolvable discriminator.mapping values (verified: 0 remaining leaks, 0 dangling #/$defs pointers, all catalog files valid JSON).
• Generation now fails fast on any future external mapping leak.

Scope / follow-ups

The original ticket cited 587 unresolved external $refs — that was already fixed on rust-port; all 1787 $refs now resolve internally. This change addresses the residual plain-string leaks. Two remaining external references are intentionally out of scope:

• GraphQL schemaRef (./graphql/schema.graphql) in catalog-products and taxes — tracked with the GraphQL commerce work (DEVX-546).
• JSON-Schema $id URIs — valid identifiers, not broken references; left as-is.

Testing

• cargo test -p generate-api-catalog — 10/10 pass (incl. 2 new)
• cargo test api_explorer — pass with rewritten catalogs embedded
• cargo clippy -p generate-api-catalog -- -D warnings — clean
• cargo fmt --check — clean

Notes for reviewers

Includes a small unrelated chore commit gitignoring legacy TS build artifacts (node_modules/, dist/, .pnpm-store/, .npmrc) that surface as untracked noise in the rust-port tree. Happy to split it out if preferred.