feat(api-core): add injectable DateFormatter + StringFormatter/Slugify

[adrians5j]

Summary

Backend counterpart to the admin-side formatting features (#5512, #5517). Brings DateFormatter, StringFormatter, and Slugify to @webiny/api-core so server code gets the same injectable, decoratable formatting.

Features (@webiny/api-core)

• StringFormatter — consumer-facing string-transform API; exposes slugify() today, more methods later. Delegates to Slugify.
• Slugify — single-method (execute()) feature holding Webiny's canonical slug options; the decorate seam behind StringFormatter.slugify().
• DateFormatter — deterministic absolute date/time formatting (UTC, YYYY-MM-DD HH:mm), decoratable. No viewer-locale dependence, unlike the admin side.

All three register with ApiCoreFeature (always available). Same shape as the admin features, using api-core's @webiny/feature/api DI. No hooks (no React) — backend consumers inject the abstraction via a feature's dependencies.

Usage

class MyRepositoryImpl {
    constructor(private stringFormatter: StringFormatter.Interface) {}

    private makeSlug(name: string) {
        return this.stringFormatter.slugify(name);
    }
}
// dependencies: [..., StringFormatter]

Override (per project) — decorate the fine-grained seam

const MySlugify = Slugify.createDecorator(() => {
    return {
        execute: value => {
            return value.trim().toLowerCase().replace(/\s+/g, "_");
        }
    };
});

container.registerDecorator(MySlugify);

Migrated

• CreateGroupRepository (api-headless-cms) now injects StringFormatter instead of importing the toSlug util.

Left as-is on purpose

• The contentModelGroup zod validation schema keeps using the toSlug util directly — it is a pure schema factory with no DI seam (mirrors the admin-side pure-util exceptions like PagePath).

Docs

• Added StringFormatter / Slugify / DateFormatter to ai-context/core-features-reference.md.

Testing

• yarn check -p @webiny/api-core / -p @webiny/api-headless-cms ✓
• contentModelGroup.crud.test.ts — 9 passed (exercises group create → CreateGroupRepository resolving StringFormatter from the container and slugifying end-to-end).
• helpers/toSlug.test.ts — 2 passed (util retained for the validation schema).
• yarn lint / yarn format / yarn adio ✓

🤖 Generated with Claude Code

[adrians5j]

/vitest

[github-actions]

Vitest tests have been initiated (for more information, click here). ✨

Group	Status
No storage	❌ 44/57 passed
DDB	✅ 17/17 passed
DDB+OS	❌ 18/19 passed
SQL	✅ 10/10 passed
PGlite	✅ 10/10 passed

❌ Failed packages

No storage

• api-file-manager-s3
• api-website-builder-workflows

[adrians5j]

Superseded by #5549, which targets next (this targeted release/6.5.0). Rebased both admin and api-core changes into that single PR.