[Streams] Terraform support POC

[rStelmach]

TL;DR

• Streams OAS / Zod has been reshaped so that Streamlang conditions, steps, ingest lifecycle, _ingest, and _group are all strongly typed in the generated Go client (no more interface{} / json.RawMessage for these parts).
• We regenerated the full kibana.yaml from this branch and confirmed that a Terraform POC can use a client generated from it via a new elasticstack_kibana_stream resource.
• On the Terraform side, we added Streams client helpers, a new stream resource schema, flatten/expand utilities, and a small internal condition DSL, proving that the new OAS is working as expected.

Background

• Previously, key parts of Streams (Streamlang conditions, processing steps, ingest lifecycle) were expressed as anonymous unions in OAS, which generated interface{}/json.RawMessage in Go.
• That made it hard to expose Streams as a strongly typed Terraform resource without custom JSON handling.
• This work makes the Streams OAS generator‑friendly and ensures the generated Go client exposes concrete types for the write APIs Terraform cares about.

Kibana: Streams + OAS changes (this PR)

• Streamlang conditions

  • conditionSchema stays a z.discriminatedUnion('type', ['filter','and','or','not','never','always']) and is tagged as @kbn/oas-component:StreamlangCondition.
  • The router‑to‑OAS converter extracts it as components.schemas.StreamlangCondition and rewrites recursive positions (and.items, or.items, not) to self‑$ref.
  • Result: in kibana.yaml / Go, all recursive condition slots are StreamlangCondition (And []StreamlangCondition, Or []StreamlangCondition, Not StreamlangCondition), not interface{}.

• Streamlang steps

  • Processor steps and where‑blocks now carry an optional kind discriminator and are wrapped into a StreamlangStep union, tagged as @kbn/oas-component:StreamlangStep.
  • The converter rewrites all processing.steps.items (including nested where.steps) to $ref: '#/components/schemas/StreamlangStep'.
  • Result: processing.steps is []StreamlangStep in Go, with proper union wrappers instead of []interface{}.

• Ingest lifecycle

  • Lifecycle moved from marker‑object unions (e.g. { dsl: {...} } | { ilm: {...} } | { inherit: {} }) to a type‑tagged discriminated union.
  • Preprocessors infer type from legacy keys on read so existing documents stay valid.
  • Result: ingest.lifecycle is a tagged union in OAS and a set of concrete structs in Go.

• GET/DELETE request bodies

  • Added isVoidRequestBodySchema to the router‑to‑OAS layer to detect the z.undefined() sentinel and drop operation.requestBody for real body‑less GET/DELETE routes.
  • Result: Streams GET /api/streams* and DELETE /api/streams* no longer have bogus request bodies, and the generated client drops the ...JSONBody = interface{} aliases and WithBody helpers for those operations.

• Full OAS regeneration

  • Enabled server.oas.enabled, captured a clean Streams‑only snapshot (for linting) and then regenerated the full OAS to oas_docs/output/kibana.yaml.
  • Verified that regenerating the Go client against this full spec preserves existing behavior while exposing the new strong types for Streams.

terraform's PR : elastic/terraform-provider-elasticstack#1473

[elasticmachine]

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