feat: add plugins exports - #4076

ℹ️ Minor suggestion — the nameBuilder type change in @hey-api/schemas isn't captured in the changesets.

Reviewed changes — adds a /plugins subpath export to both @hey-api/openapi-ts and @hey-api/openapi-python, backed by a new pluginHelper() factory in @hey-api/shared that produces type-safe plugin config builder functions. Internal dev presets are migrated to use the new helpers.

• Add pluginHelper() in @hey-api/shared — a generic factory that returns a curried plugin config builder, stripping name from the user-facing parameter and narrowing it to the specific plugin name literal in the return type.
• Add /plugins subpath exports — new package.json exports entries ("./plugins") and tsdown entry points for both openapi-ts and openapi-python, each exposing a plugins.ts barrel that exports one helper per registered plugin.
• Migrate dev presets to plugins helpers — dev/typescript/presets.ts and dev/python/presets.ts switch from raw plugin name strings/config objects to plugins.myPlugin({ ... }) calls.
• Clean up @hey-api/schemas types — drops the @hey-api/spec-types import and widens nameBuilder's schema callback parameter from a union of specific OpenAPI types to unknown.

ℹ️ nameBuilder callback type narrowed to unknown without changelog entry

The @hey-api/schemas plugin's nameBuilder callback parameter schema changed from OpenAPIV2.SchemaObject | OpenAPIV3.ReferenceObject | OpenAPIV3.SchemaObject | OpenAPIV3_1.SchemaObject to unknown. This is a backward-incompatible type change for callers that access schema properties inside their nameBuilder callback.

The three changesets cover the plugin exports feature but don't mention this type change. Consider adding a short note to one of the @hey-api/openapi-ts patch changesets.

# nameBuilder schema type widened to unknown

## Affected sites
- `packages/openapi-ts/src/plugins/@hey-api/schemas/types.ts:13` — `schema` parameter changed from OpenAPI union to `unknown`

## Required outcome
- Add a changeset note documenting the `nameBuilder` schema type change so consumers are aware.

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