fix(ui5-list): add missing accessible description for growing button

[NakataCode]

• Extended TypeScript interface: Added description? property to ListAccessibilityAttributes.growingButton
• Added getter method: Implemented growingButtonAriaDescribedBy() that returns the description element ID when description is provided
• Updated template: Added aria-describedby attribute and conditional hidden span with ui5-hidden-text class
• Updated documentation: Enhanced JSDoc to document the new description property
• Added Cypress test
• Added test sample in List.html for manual verification

---

Example Usage:

list.accessibilityAttributes = {
  growingButton: {
    name: "Load more products",
    description: "This will load additional items from the catalog"
  }
};

---

Fixes: #12443

[ui5-webcomponents-bot]

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