Added component testing fixtures and integration coverage

[josiahking]

Summary

Adds reusable component testing fixtures to evolvephp/testing and expands integration coverage around module/plugin startup behavior.

What changed

• Added ComponentDefinitionFixture for creating lightweight component definitions in tests.
• Added ComponentEntryPointFixture for callback-driven lifecycle testing.
• Added unit coverage for both fixtures.
• Added an integration test covering:
  • explicit module definitions
  • Composer plugin discovery
  • component enablement
  • dependency ordering
  • service registration
  • container freeze before boot
  • boot and ready callbacks
  • reverse shutdown ordering
• Updated the Testing package documentation.
• Updated package documentation and source inventory.

Validation

• Full test suite: 499 tests, 1981 assertions
• Architecture and documentation checks: 193 tests, 7501 assertions
• Static analysis passed
• Dependency-boundary checks passed
• Coding-style checks passed
• Security and licence checks passed
• Package release validation passed
• Package split validation passed
• Consumer validation passed

Package impact

Only evolvephp/testing changes in the package split. The Contracts, Core, Module, Plugin and HTTP package splits remain unchanged.