Fix TypeScript index signatures for mixed properties

[yhuikzdtguioaert]

Fixes #3123.

What changed

TypeScript now widens a typed additionalProperties index signature with the declared property types. If any declared property is optional, the index signature also includes undefined, which is required by tsc --strict.

The ordinary unconstrained additionalProperties path remains unchanged (any/unknown), so common generated output does not gain redundant unions. Flow keeps its existing rendering through the base hook.

A JSON Schema fixture includes both a valid mixed object and an invalid additional property, covering generation, strict compilation, successful conversion, and runtime rejection.

Verification

• npm run build — passed
• generated --just-types output + tsc --ignoreConfig --noEmit --strict — passed for default, --prefer-types, and --prefer-unknown
• generated converter positive round-trip and negative schema rejection — passed
• scoped biome check for all changed source/fixture files — passed
• Vitest on Windows: 230 passed; 6 existing platform-specific path/spawn tests failed outside the changed renderer