feat(Campaign): make context creation extensible via overridable template method

[timowestnosto]

Problem
NostoCampaign currently calls getContext(...) directly when preparing template data. This prevents subclasses or future variations (e.g., adding custom context enrichment, feature flags, A/B metadata) from extending or modifying the context before rendering.

Goal
Introduce a template method that centralizes context construction so it can be overridden or monkey‑patched without duplicating render logic.

Proposal

1. Add a method (e.g. createContext(raw: JSONResult): object) in NostoCampaign.
2. Replace direct getContext(...) usage with this.createContext(...).
3. Document the extension point in TypeDoc.

Acceptance Criteria

• Campaign still renders identically (no behavioral changes).
• Tests pass (no snapshot/content change).
• Subclasses can override createContext to inject/transform values.
• TypeDoc shows the new method.
• No lint/type errors.

Non-Goals

• No refactor of getContext itself.
• No changes to other components yet.

Risks

• Overridden implementations might skip required normalization. Mitigate via doc note: “Call super.createContext(raw)” or reuse getContext.

Suggested Patch (illustrative)

// ...existing code...
import { getContext } from "../../templating/context"
// ...existing code...

export class NostoCampaign extends NostoElement {
  // ...existing code...

  /**
   * Extension point: override to enrich or modify the template context
   */
  // eslint-disable-next-line @typescript-eslint/no-explicit-any
  createContext(raw: JSONResult) {
    return getContext(raw)
  }

  async #renderRecommendation(rec: Recommendation) {
    // ...existing code...
    // before:
    // const ctx = getContext(base)
    const ctx = this.createContext(base)
    // ...existing code...
  }
  
  // ...existing code...
}

Follow-up Tasks

• Add a brief test with a subclass overriding createContext (optional but recommended).
• Update README / developer docs (extension section) if such section exists.

Labels Suggested:
type: enhancement, component: campaign, extensibility, good first issue

Let me know if you want the test scaffold added in the same issue.