fix(openapi): JSON Schema for single-constructor records and generic declarations

[DamianReeves]

Part of finos/morphir#958, phase B.

What

The MEP contract needs two changes in the morphir-openapi JSON Schema backend:

• Write a single-constructor custom type as a plain object. Today it becomes a oneOf with a kind discriminator, which does not match the MEP wire shape.
• Accept the generic declarations the contract uses. Today they fail with JSC003.

Done when

• A Gleam record type produces a plain object schema.
• A generic declaration used with concrete arguments produces a schema.

Design

checkpoint 9 in finos/morphir#949.

[DamianReeves]

Tracking moved to beads: morphir-opua.1.2.3 in the finos/morphir repository's .beads database. This issue stays open as a mirror and links there.

[DamianReeves]

Scope note from a beads audit (2026-09-25): the MEP contract needs only the non-generic part of this issue. That part stays in this issue's bead (morphir-opua.1.2.3). Support for generic declarations is now tracked separately in bead morphir-opua.1.2.9, at lower priority.