Add behat test for config has after concatenation assignment

[pwtyler]

Summary

• Adds a behat scenario to config-has.feature that verifies wp config has finds variables and constants when the wp-config.php contains a variable assignment with string concatenation (e.g. $x = 'foo' . $bar;)
• This test will fail until Remove DOTALL flag from parse_wp_config() regexes wp-config-transformer#64 is merged and released then dep bumped over here.

Depends on wp-cli/wp-config-transformer#64

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces a new test scenario to verify that the wp config has command correctly identifies variables and constants when they follow a concatenation assignment. A review comment points out that the wp-config.php content in the test is missing the required <?php opening tag, which is necessary for the PHP parser to correctly process the file and for the test to function as intended.