Add support for parsing w:sym tags in Word2007 reader

[Copilot]

This PR adds support for parsing w:sym (symbol) tags in DOCX files, which was previously ignored by PHPWord's Word2007 reader.

Problem

When reading DOCX files containing symbol elements (like special characters from Wingdings or Symbol fonts), PHPWord would skip these w:sym tags entirely, resulting in missing content in the parsed document.

Solution

Added handling for w:sym nodes in the readRunChild() method of AbstractPart.php. The implementation:

1. Extracts the w:font and w:char attributes from w:sym elements
2. Formats them as readable text in the format [{font},{char}]
3. Adds the formatted text to the document while preserving font and paragraph styling

Example

A DOCX file containing:

<w:r>
    <w:t>XXXX </w:t>
</w:r>
<w:r>
    <w:sym w:font="Wingdings 2" w:char="00A3"/>
</w:r>

Will now produce the text output:

XXXX [Wingdings 2,00A3]

Changes

• Core functionality: Added 6 lines to handle w:sym nodes in AbstractPart.php
• Test coverage: Added comprehensive test testReadSymbol() to verify the functionality works correctly with multiple symbols and edge cases
• Edge cases: Handles missing or empty attributes gracefully

The implementation is minimal and surgical, adding only the necessary functionality without modifying existing code paths.

Fixes #2802.

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[coveralls]

coverage: 96.757%. remained the same
when pulling 1904d0e on copilot/fix-2802
into 0ab0b49 on master.