[refactor] type safety and edge case tests

[ordinary9843]

Summary

• Add declare(strict_types=1) to all src/ files
• Replace __call delegation with explicit typed proxy methods in Ghostscript.php
• Replace string-concat class lookup in HandlerFactory with explicit map array
• Fix nullable type annotations (BaseException $previous → ?BaseException $previous)
• Fix wrong docblock in ConfigTrait (@param int $tmpPath → @param string $tmpPath)

Bug fixes

• SplitHandler: range(0, N-1) returned part_0.pdf…part_{N-1}.pdf but GS outputs part_1.pdf…part_N.pdf
• MergeHandler / GetTotalPagesHandler: $file undefined in catch block if exception thrown before assignment
• MergeHandler / SplitHandler / ToImageHandler: missing parent::__construct() call
• BaseHandler::isPdf(): calling mime_content_type() on non-existent file emitted E_WARNING
• MergeHandler: empty $files after filtering caused GS to hang; now throws HandlerException
• ToImageHandler: invalid image type silently passed to GS; now validated against allowed types

New edge case tests (+34 tests, 123 total)

• BaseHandlerEdgeCaseTest — empty string / non-existent path / directory path, tmp file ops, options format
• ConvertHandlerEdgeCaseTest — empty file path
• GuessHandlerEdgeCaseTest — no version header, empty path
• MergeHandlerEdgeCaseTest — empty array, all-invalid files, single valid file
• SplitHandlerEdgeCaseTest — verifies output starts at part_1.pdf, files actually exist
• ToImageHandlerEdgeCaseTest — invalid image type (bmp) throws exception
• GetTotalPagesHandlerEdgeCaseTest — empty path, non-existent file
• HandlerFactoryEdgeCaseTest — getTotalPages handler, case-sensitive type check
• PathHelperEdgeCaseTest — empty string, backslashes, mixed separators, spaces in path

Workflow

• Add pull_request trigger to build.yml so CI runs on PRs across PHP 7.1–8.4

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