Add cleanup configuration schema

Problem/Motivation

The module introduces configurable orphan cleanup settings, but currently there is no configuration schema defined for the block_content_explorer.settings configuration object.

Without a schema definition, Drupal cannot properly validate, translate, or export configuration values. This may also produce configuration validation warnings during configuration management operations.

Steps to reproduce

1. Add configuration values to block_content_explorer.settings
2. Run configuration validation commands
3. Observe missing schema warnings for module configuration

Proposed resolution

• Create config/schema/block_content_explorer.schema.yml
• Add schema definition for block_content_explorer.settings
• Define schema types and labels for:
  • auto_cleanup_enabled (boolean)
  • cleanup_interval_days (integer)
  • max_blocks_per_cron (integer)
• Ensure configuration exports and validation work correctly

Remaining tasks

• Create schema configuration file
• Add schema definitions for cleanup settings
• Validate configuration schema using Drush
• Verify configuration export/import behavior

User interface changes

None.

API changes

Adds configuration schema support for:

block_content_explorer.settings

Data model changes

None.

This issue only adds configuration schema metadata and does not introduce database schema changes.