Linting tools: add PHPStan

[jeherve]

Changes proposed in this Pull Request:

- Linting: add PHPStan tools to analyze the codebase

• Avoid issues with custom apply_filters
• Fix level 4 issues
• Add PHPStan to linting action

A few notes:

• When trying to use PHPStan's options to ignore specific lines, I kept getting memory fatal errors. So I had to make those ignore rules in the config file instead.
• When trying to up the config to level 5, I keep running into the same memory Fatals locally. I may try it in the CI though.

Edit: I see I get the same fatals in the CI already, even at level 4:

PHP Fatal error:  Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in phar:///home/runner/work/posts-on-this-day/posts-on-this-day/vendor/phpstan/phpstan/phpstan.phar/vendor/nikic/php-parser/lib/PhpParser/Lexer.php on line 263
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in phar:///home/runner/work/posts-on-this-day/posts-on-this-day/vendor/phpstan/phpstan/phpstan.phar/vendor/nikic/php-parser/lib/PhpParser/Lexer.php on line 263
Script ./vendor/bin/phpstan analyze -c phpstan.neon.dist --memory-limit 128M handling the phpstan event returned with error code 255

Testing instructions:

• Do the tests pass?

[Copilot]

Pull request overview

This PR adds PHPStan static analysis tooling to the WordPress plugin codebase to improve code quality and catch potential issues. The implementation includes PHPStan at level 4 with WordPress-specific extensions and integrates it into the existing CI/CD pipeline.

• Adds PHPStan 2.1.33 along with WordPress-specific extensions for static analysis
• Fixes PHPStan level 4 issues by adding missing return type documentation
• Integrates PHPStan into GitHub Actions workflow with memory limits to handle analysis requirements

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
phpstan.neon.dist	New PHPStan configuration file setting level 4 analysis for src and tests directories
composer.json	Adds PHPStan dependencies and creates phpstan script with memory limit configuration
composer.lock	Updates lock file with PHPStan and WordPress stubs dependencies
src/class-posts-on-this-day-widget.php	Adds @return void and @return string|void documentation for widget methods
.github/workflows/lint.yml	Adds new phpstan job to run static analysis in CI with 5-minute timeout
.gitattributes	Adds phpstan.neon.dist to export-ignore list and reformats for consistency

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The phpstan script includes a composer update command which may cause unexpected dependency updates during static analysis. PHPStan should run against the current dependency state without updating packages. Consider removing the "@composer update" line to avoid unintended side effects during static analysis runs.

Suggested change

"@composer update",