feat: add hide_content_field config to ElementEmbeddedCode

[jsirish]

Summary

Adds a hide_content_field config flag (default false) that removes the Content field from the ElementEmbeddedCode CMS edit form, leaving only the embed code field.

This reimplements #15 (which targeted the deprecated master branch and used a hardcoded ElementEmbeddedCode::config()) on the active 4 branch, using static::config() so subclasses can override the flag via late static binding.

Usage

---
After:
  - '#silverstripeelemental-embedded-codeconfig'
---
Dynamic\Elements\Embedded\Elements\ElementEmbeddedCode:
  hide_content_field: true

Replaces #15.

🤖 Generated with Claude Code

[jsirish]

PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🎫 Ticket compliance analysis ✅ 15 - Fully compliant Compliant requirements: Add a config option to hide the Content field from ElementEmbeddedCode CMS edit form Config option should be named hide_content_field Default behavior should keep Content field visible (opt-in to hide) Should work with SilverStripe's config system

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ No major issues detected