Add strict slug validation for plugin-tests and theme-tests commands

[Copilot]

When users pass plugin/theme names with invalid characters to scaffold plugin-tests or scaffold theme-tests, the generated bootstrap.php contains corrupted file paths like '/my-plugin/.php', causing PHP fatal errors on test execution.

Changes

• Strict Validation: Added regex validation in scaffold_plugin_theme_tests() to only allow alphanumeric characters (a-z, A-Z, 0-9) and dashes (-)
• Error handling: Shows clear error message instead of silently creating corrupted files
• Test coverage: Added Behat scenarios for both plugin-tests and theme-tests commands to verify rejection of invalid characters

Example

// Before: wp scaffold plugin-tests my-plugin/
// Generated: require dirname( dirname( __FILE__ ) ) . '/my-plugin/.php';  ❌

// After: wp scaffold plugin-tests my-plugin/
// Error: Invalid plugin slug specified. The slug can only contain alphanumeric characters and dashes.

The validation rejects:

• Slashes: my-plugin/ or my-plugin\
• Underscores: my_plugin
• Special characters: my@plugin, my.plugin
• Spaces: my plugin
• Parent directory references: ../plugin

The fix follows the existing pattern for invalid slug validation (. and ..) and implements the hard-error approach rather than silently transforming input.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Adding slashes at the end of plugin name on 'plugin-tests' subcomand generates corrupted bootstrap.php</issue_title>
<issue_description>Adding one or more slashes at the end of plugin name like for example:

%> wp-cli scaffold plugin-tests my-plugin/

runs right, but causes the creation of a corrupted bootstrap.php file that produces a PHP Fatal error on require() that file when launching tests.
(The usual message is like following: "PHP Fatal error: require(): Failed opening required '/path/to/my-plugin/.php' (include_path='.') in /path/to/my-plugin/tests/bootstrap.php on line 26.")

I'd like to send a PR in order to fix this for example by right trimming slashes to $slug before to send to mustache template.
</issue_description>

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

@danielbachhuber > I'd like to send a PR in order to fix this for example by right trimming slashes to `$slug` before to send to mustache template.

What do you think about hard-erroring instead? I'm not sure it makes sense to magically transform user input in this case; probably better to inform the user they've provided invalid input (as we've done in #26)</comment_new>
<comment_new>@danielbachhuber

So I'd like to send a PR very soon.

Sounds good! I'll defer to your judgement on what changes make sense.</comment_new>

• Fixes Adding slashes at the end of plugin name on 'plugin-tests' subcomand generates corrupted bootstrap.php #133

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