(POC) feat: Add composable component support

[mokhld]

This spike replaces the hardcoded instructionText logic with a more flexible composition pattern, letting components include other components via before and after properties.

Sister PR DEFRA/forms-designer#1164

Changes

forms-model
• types.ts: Added ComposableComponent interface with before and after props; extended ComponentDef accordingly
• form-definition/index.ts: Updated schema to allow before and after (using Joi.any() for spike simplicity — recursive validation needed later)

forms-engine-plugin
• ComposableComponentCollection.ts: New class extending ComponentCollection to support before/after relationships
• component-renderer.ts: New helper for recursively rendering composable components while preserving type integrity
• QuestionPageController.ts: Detects composable components and switches to ComposableComponentCollection, otherwise defaults to the regular collection

How It Works
1. Components can define before and after subcomponents
2. Engine detects composable components and uses the enhanced collection
3. Rendering order: before → main → after
4. Fully backward compatible — existing forms continue to work

{
  "type": "EmailAddressField",
  "name": "email",
  "title": "Email address",
  "before": {
    "type": "InsetText",
    "content": "Important: We'll only use this to send confirmation"
  },
  "after": {
    "type": "Details",
    "title": "Why we need your email",
    "content": "Your email helps us send confirmations..."
  }
}

[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 90%)
3 New Major Issues (required ≤ 0)

See analysis details on SonarQube Cloud

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