Upgrade Symfony dependencies and enhance CI with scoped service locator

[bpteam]

This pull request introduces major changes for version 3.0.0 of the bundle, focusing on upgrading dependencies, improving Symfony and PHP compatibility, and enhancing developer tooling and documentation. The most important changes are grouped below.

Dependency and Compatibility Updates:

• Dropped support for PHP 7.4 and now require PHP 8.3; updated composer.json to require Symfony 6.4, 7, and 8, and upgraded the base library to axtiva/flexible-graphql-php version 3.0.
• Added support for Symfony 8 and PHP 8.3; removed support for older Symfony and PHP versions. [1] [2]

Developer Tooling and Testing:

• Introduced a new GitHub Actions CI workflow (.github/workflows/ci.yml) that tests against Symfony 6.4, 7, and 8 with PHP 8.3, runs static analysis with PHPStan, and executes integration/code generation tests.
• Added PHPStan level 6 configuration and upgraded PHPUnit to version 12 for improved code quality and testing. [1] [2]

Documentation and Example Updates:

• Updated documentation to reflect new installation requirements, removed outdated example project references, and clarified the use of the scoped TypeRegistry locator in Symfony service configuration. [1] [2] [3]
• Added a knowledge base entrypoint and agent rules in AGENTS.md to guide repository navigation for documentation and skills discovery.
• Added a detailed architecture overview in docs/index/architecture.json describing the bundle's structure, configuration model, and integration boundaries.

Changelog:

• Documented all breaking changes and new features in CHANGELOG.md under version 3.0.0.