Fix flush command bugs and expose clear-all cache for deployments

[Copilot]

• Fix duplicate wp_cache_post_change() call bug in flush command
• Add success messages for --post_id and --permalink flush cases
• Update flush command to use modern WP-CLI docblock format with ## OPTIONS and ## EXAMPLES
• Fix --post_id validation to use absint() and check > 0 (rejects 0, negatives, non-numeric strings)
• Fix --permalink validation to use absint() and check > 0 (url_to_postid() returns 0 on failure, is_numeric(0) was true)
• Add Behat test scenarios for flush (all cache), flush --post_id, invalid --post_id
• Add Behat test scenarios for flush --permalink (success) and unknown permalink (error)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add a clear all setting</issue_title>
<issue_description>WP Super Cache has a clear all capability in the plugin.

When doing code pushes having the ability to clear all the cache would be extremely helpful.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Add a clear all setting #15

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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