feat(openapi-v3): Preserve JSON Schema const values as TypeScript literal types

[sandovalrr]

Summary

This PR preserves JSON Schema / OpenAPI const constraints as TypeScript literal types in generated output.

Before this change, schemas such as { type: "string", const: "READY" } were widened to string because the shared schema emission path did not recognize const. As a result, generated unions and discriminator-like object branches lost useful literal information.

With this change, supported scalar const values are emitted as their corresponding TypeScript literal types:

• string -> "VALUE"
• number -> 123
• boolean -> true / false
• null -> null

This applies consistently anywhere schemas are emitted through the existing generator flow, including nested properties and composed schemas.

Root Cause

The shared generator logic in schemaToTypeAliasDeclaration.getType() handled:

• $ref
• oneOf / anyOf / allOf
• enum
• primitive schema types

but it never handled const.

That meant a schema with const fell through to the normal primitive branches and was widened to string, number, or boolean instead of preserving the literal value.

Implementation

The fix adds explicit scalar-const handling in the generator’s shared schema-to-TypeScript emission path.

Key points:

• const is handled centrally in schemaToTypeAliasDeclaration.ts
• supported scalar values are converted to TypeScript literal type nodes
• nullable: true continues to work for non-null const values
• composed schemas continue to work because existing recursive composition logic now naturally picks up const in branches
• existing enum, $ref, and non-const behavior is unchanged

This keeps the change small and aligned with the current architecture.

Tests

Added focused coverage for:

• top-level string / number / boolean / null const
• nullable const
• oneOf / anyOf branches with const
• allOf with compatible const
• object oneOf branches with const discriminator-like properties and mixed value types
• generator-level regression coverage proving const emits literal types while adjacent non-const schemas still generate as before

Scope / Tradeoffs

This PR intentionally supports scalar const values only. Object and array const values are left out of scope to keep the change narrow, maintainable, and reviewer-friendly.