Harden current_tab() by sanitizing request input

[Ankita19Rathore]

What this PR does

This PR hardens the current_tab() method in the Health Check plugin.

Previously, the method returned the value of $_GET['tab'] directly when present.
This update makes the method more defensive by safely handling missing input and sanitizing user-controlled request data.

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What was changed

Before

return ( isset( $_GET['tab'] ) ? $_GET['tab'] : 'site-status' );

After

if ( empty( $_GET['tab'] ) ) {
	return 'site-status';
}

return sanitize_key( wp_unslash( $_GET['tab'] ) );

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Why this change is important

The tab parameter is derived from request data and may be user-controlled.
Sanitizing this input improves robustness, avoids unexpected values, and aligns the code with WordPress security and coding best practices.

This is a defensive hardening change; existing behavior remains unchanged for valid input.

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Scope and safety of change

• Change is limited to the current_tab() method

• No UI or functional behavior changes

• Fully backward-compatible

• Low-risk improvement

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Additional cleanup

Updated the class docblock to correctly reflect the Health_Check class name

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Testing

Verified default behavior when no tab parameter is present

Verified valid tab values continue to work as expected