fix(diagnostics): declare $cleanDiagnostics property

[faisalahammad]

Summary

Fixes the PHP 8.3 deprecation notice emitted on every diagnostic AJAX request: Deprecated: Creation of dynamic property PostmanGetDiagnosticsViaAjax::$cleanDiagnostics is deprecated. The class assigned to the property in its constructor without declaring it, so PHP 8.3 flags it and PHP 8.4 would escalate it to a fatal error.
\nFixes #126

Changes

Postman/Postman-Diagnostic-Test/PostmanDiagnosticTestController.php

Before:

class PostmanGetDiagnosticsViaAjax {
    private $diagnostics;
    private $options;
    private $authorizationToken;

After:

class PostmanGetDiagnosticsViaAjax {
    private $diagnostics;
    private $cleanDiagnostics;
    private $options;
    private $authorizationToken;

Why: The constructor at line 170 assigns $this->cleanDiagnostics = ''; without a matching declaration, so every call creates the property dynamically. Adding the declaration next to its siblings silences the deprecation without changing behaviour. All existing usage at lines 170, 176, and 348 still works.

Testing

PHP 8.3 + WP_DEBUG

1. Install Post SMTP 3.3.0 with PHP 8.3, WP 6.8.1, and WP_DEBUG=true WP_DEBUG_LOG=true.
2. Before applying the fix: trigger the diagnostic test from wp-admin and confirm wp-content/debug.log contains the "Creation of dynamic property" deprecation line.
3. Apply the fix and repeat step 2.
4. Result: the deprecation entry no longer appears, and the post_smtp_clean_diagnostic_report_data option is still populated with the diagnostic text as before.
   \nTested manually against a WordPress 6.8.1 install on PHP 8.3.