Feature/php8.3 support

[emgiezet]

Summary

This PR adds full PHP 8.3 support to errbit-php and significantly improves test coverage from ~73% to ~88%.

Key Changes

PHP 8.3 Compatibility Fixes:

• Updated Dockerfile to use php:8.3-fpm-alpine with PECL-based Xdebug installation
• Fixed WriterInterface to use \Throwable instead of removed ErrorInterface
• Added proper constructor signatures to error classes (BaseError, Notice, Warning, Error, Fatal) with $file and $backtrace parameters
• Fixed ErrorHandlers::onShutdown() parameter order for Fatal constructor
• Fixed ErrorHandlers::onException() to pass correct arguments to Converter::convert()
• Fixed Errbit::shouldNotify() null access issue with ignore_user_agent
• Replaced deprecated (integer) cast with (int) in Converter.php
• Fixed print_r() second parameter to use bool instead of int (PHP 8.3 strict)
• Removed unused imports across multiple files
• Deleted obsolete ErrorInterface.php

Test Coverage Improvements (+1200 lines of tests):

• Extended ErrbitTest.php with 15+ new tests (singleton, configure, onNotify, shouldNotify, ignore_user_agent)
• Extended ErrorHandlersTest.php with tests for onError, onException, different error types
• Created BaseErrorTest.php for error class constructors
• Extended NoticeTest.php with ~25 tests for XML generation, filtering, formatting
• Extended XmlBuilderTest.php with XmlBuilder-specific tests
• Created SocketWriterTest.php with connection scheme and payload tests
• Extended GuzzleWriterTest.php with async/sync write tests

CI/CD Updates:

• Updated GitHub Actions workflow for PHP 8.2 and 8.3 support

Test Plan

• Run XDEBUG_MODE=coverage php bin/phpunit --testsuite UnitTests --coverage-text
• Verify 108 unit tests pass
• Verify line coverage is ~88%
• Verify all error classes work with PHP 8.3 strict types