fix(elixir): decode discriminated inline unions without a mapping

[calvin-archastro]

Review on ArchCode

Problem

Regenerate SDK in archastro-elixir (#24) fails the ExternalObjects list contract test:

spec:      ExternalObject.object = oneOf [inline r2_bucket, inline d1_database], discriminator.propertyName "type", no mapping
prepare:   mapping inferred from $ref variants only → {} (inline variants have no schema name)
descriptor: `mapping ?? inferred` → {} is not nullish → {:union, [...], {"type", %{}}}
Codec:     mapping["r2_bucket"] → nil → matches? false
result:    ArgumentError "response does not match required fields for …GetApiV1ExternalObjects.Response"

The same empty-mapping descriptor was also generated in agents.ex, ai.ex, and api_chat_channel.ex. Other languages don't use a tag→module mapping, so only Elixir is affected.

What changed

• backends/elixir/index.ts descriptor(): if the explicit or inferred mapping has no entries, emit {:union, variants} instead of a discriminated union with %{}. Codec.decode/2 then tries each variant. Each hoisted variant struct requires its own type enum value, so exactly one matches.
• Discriminated unions with a real mapping (explicit, or inferred from $refs) are unchanged. Existing test preserves original implicit discriminator tags while normalizing modules still passes.

Scope / risk

Generator only (Elixir). Unchanged: every union that currently decodes. Changed: unions that currently fail on every input. Low risk.

Testing

• New elixir.test.ts › decodes discriminated inline union variants without a mapping by trying each variant: failed before the fix (output contained {"type", %{}}), passes after.
• npm test: 426/426. tsc --noEmit and npm run lint clean.
• End-to-end: regenerated archastro-elixir (auto/regenerate-sdk) with this build via ARCHASTRO_SDK_GENERATOR_BIN=… ./scripts/regenerate_sdk.sh --local <this checkout>, then ran CI's steps locally:
  • mix format --check-formatted ✅, mix compile --warnings-as-errors ✅, mix dialyzer → 0 errors.
  • ExternalObjects contract suite against Prism: 27/27 (previously 26/27).
  • mix test test/contract --only channel_contract with the harness from this repo: 16/16.
• Automatic coverage: this repo's CI runs the vitest case. archastro-elixir CI runs the generated contract tests on each regen PR.

Follow-up

Release sdk-generator v0.11.11, rerun Regenerate SDK in archastro-elixir, then update that PR's hand-written fixtures and its channel-harness lockfile entry (0.5.0 → 0.7.1). That lives in the Elixir repo, not here.

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