fix(controlplane): mark arbitrary-JSON object fields as open maps in OpenAPI spec

[mekilis]

Summary

Follow-up to #2736. That PR fixed json.RawMessage fields from integer arrays to type: object, but a bare {type: object} with no properties/additionalProperties is a closed empty object. Strict SDK generators produce key-stripping schemas from it — the TypeScript client's zod schema serialized every event data payload as {}:

modelsCreateEventToJSON({ data: { amount: 100 }, ... })
// => {"data":{},"endpoint_id":"ep1","event_type":"invoice.paid"}

• docs/fix_openapi_spec.sh gains a doc-gen pass that sets additionalProperties: true on bare-object properties (event data, metadata data, json_schema, bulk-filter bodies). Typed maps like custom_headers (additionalProperties: {type: string}) are untouched.
• Regenerated docs/.

Test plan

• make generate_docs clean; openapi spec validate passes
• Spec audit: all arbitrary-JSON fields now additionalProperties: true; typed maps unchanged; handlers.Stub (whole-definition stub) unchanged
• Empirically confirmed the key-stripping on the current TS client build (zod z.object({}) strips all keys)

[cursor]

Current version of PR was reviewed by /review-bugbot on Jul 19, 21:32 GMT+1. It flagged 0 findings.

^{Bugbot on commit 1faad17 is skipped.}