Missing config schema for module settings causes config validation issues

Problem/Motivation

The module defines configuration (debug_cacheability_headers_split.settings) but does not provide a corresponding schema definition.

Without a schema:

• Drupal cannot properly validate configuration
• Configuration translation does not work correctly
• Strict config schema checks (e.g. during config import or in CI) may fail
• It may trigger warnings in tools like drupal:config:validate or during runtime in strict environments

Steps to reproduce

• Install and enable the module
• Export configuration (drush cex) or run config validation
• Observe missing schema warnings for debug_cacheability_headers_split.settings

Proposed resolution

Add a configuration schema file defining the structure of debug_cacheability_headers_split.settings, including:

• header_size_limit (integer)
• header_chunk_size (integer)

This ensures proper validation, improves compatibility with config translation, and aligns with Drupal best practices.