refactor: Require php 8.4 and upgrade pest

[tymondesigns]

This pull request requires PHP 8.4+, upgrades the test toolchain to Pest 5, and aligns CI, static analysis, and docs with that baseline.

PHP version and dependencies:

• Require PHP 8.4+ in composer.json, README, docs, and CI. PHP 8.3 is no longer tested or supported.
• Upgrade to Pest 5 and the matching plugins (phpstan, rector, type-coverage).
• Pin pestphp/pest-plugin-mutate to ^5.0.1 so prefer-lowest CI cannot install 5.0.0, which crashes mutation testing against PHPUnit's --coverage-php array dump.

Tooling:

• ECS targets PHP 8.4; Rector uses the PHP 8.4 set plus Pest coding style.
• PHPStan now analyzes tests via pest-plugin-phpstan instead of a custom expectation stub.
• composer test runs Pest in parallel.
• CI matrix is PHP 8.4/8.5 with fail-fast: false.

Tests:

• Bootstrap uses Pest 5 pest()->extend().
• OpenAPI validation assertions are helper functions (assertOpenApiValidationPasses, assertOpenApiValidationErrors, assertOpenApiValidationFailsAt) rather than expect()->extend() matchers, so PHPStan and the IDE can follow them without stubs.

[Copilot]

🟢 Approval recommended

The migration is internally consistent, and the complete test, mutation, formatting, PHPStan, and type-coverage matrix passes.

Pull request overview

Raises the project baseline to PHP 8.4 and modernizes testing and analysis for Pest 5.

Changes:

• Updates PHP requirements, dependencies, CI, and documentation.
• Migrates Pest configuration and validation assertions to typed helpers.
• Applies PHP 8.4/Pest formatting and expands PHPStan analysis to tests.

File summaries

File	Description
composer.json	Updates PHP and Pest dependencies and parallel testing.
.github/workflows/run-tests.yml	Tests PHP 8.4/8.5 across platforms and stability levels.
.github/actions/setup-php-composer/action.yml	Defaults setup to PHP 8.4.
README.md	Documents PHP 8.4 requirement.
docs/openapi/introduction.mdx	Updates documented PHP baseline.
docs/openapi/installation.mdx	Updates installation requirements.
phpstan.dist.neon	Enables Pest integration and test analysis.
rector.php	Targets PHP 8.4 and Pest style.
ecs.php	Enables PHP 8.4 migration rules.
tests/Pest.php	Uses the Pest 5 bootstrap API.
tests/Expectations.php	Replaces custom expectations with typed helpers.
tests/phpstan/pest-expectations.stub	Removes the obsolete expectation stub.
src/OpenApi.php	Applies PHP 8.4 constructor dereferencing.
src/Concerns/BuildsArray.php	Adds targeted PHPStan suppressions.
tests/Unit/IntegrationTest.php	Adds typed array narrowing and chained assertions.
tests/Unit/OpenApiTest.php	Modernizes assertions for Pest 5 analysis.
tests/Unit/Concerns/BuildsArrayTest.php	Adopts PHP 8.4 syntax and chained assertions.
tests/Unit/Concerns/HasExtensionsTest.php	Adopts PHP 8.4 syntax and removes redundant documentation.
tests/Unit/Objects/ComponentsTest.php	Modernizes empty-array assertion.
tests/Unit/Objects/ExampleTest.php	Modernizes empty-array assertions.
tests/Unit/Objects/ExternalDocsTest.php	Chains related assertions.
tests/Unit/Objects/HeaderTest.php	Modernizes empty-array assertion.
tests/Unit/Objects/LinkTest.php	Uses typed key-position helpers and chains assertions.
tests/Unit/Objects/MediaTypeTest.php	Uses typed key-position helpers and chains assertions.
tests/Unit/Objects/OAuthFlowsTest.php	Modernizes empty-array assertion.
tests/Unit/Objects/OperationTest.php	Chains method and callback assertions.
tests/Unit/Objects/PathItemTest.php	Narrows list values to arrays.
tests/Unit/Objects/ReferenceTest.php	Consolidates component shortcut assertions.
tests/Unit/Objects/RequestBodyTest.php	Narrows response content to an array.
tests/Unit/Objects/ResponseTest.php	Modernizes and narrows response assertions.
tests/Unit/Objects/SecurityRequirementTest.php	Modernizes empty-array assertion.
tests/Unit/Objects/XmlTest.php	Modernizes empty-array assertion.
tests/Unit/Validation/ComponentsTest.php	Uses typed validation helper.
tests/Unit/Validation/DocumentTest.php	Uses typed validation helpers.
tests/Unit/Validation/ExampleTest.php	Uses typed validation helper.
tests/Unit/Validation/LicenseTest.php	Uses typed validation helper.
tests/Unit/Validation/ParameterTest.php	Uses typed validation helpers.
tests/Unit/Validation/PathsTest.php	Uses typed validation helper.
tests/Unit/Validation/RequestBodyTest.php	Uses typed validation helpers.
tests/Unit/Validation/ResponseTest.php	Uses typed validation helpers.
tests/Unit/Validation/SecuritySchemeTest.php	Uses typed validation helpers.
tests/Unit/Validation/ValidationExceptionTest.php	Uses PHPUnit failure assertions and array narrowing.

Review details

• Files reviewed: 42/42 changed files
• Comments generated: 0
• Review effort level: Balanced

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