…toring_factory (#276)

* refactor(factories): remove unnecessary di_container_factory and monitoring_factory

Remove redundant factory implementations as part of C++20 module preparation:

- Remove public API factories (include/kcenon/logger/core/di/, monitoring/)
  - di_container_factory.h: simple wrapper with unused container types
  - di_container_interface.h: non-template interface never used
  - monitoring_factory.h: embedded simple_monitor class, redundant

- Remove internal factories (src/impl/di/, monitoring/)
  - di_container_factory.h: thread_system fallback logic never invoked
  - monitoring_factory.h: thread_system fallback logic never invoked

- Remove unimplemented DI methods from logger.h:
  - set_di_container(), has_di_container()
  - add_writer_from_di(), register_writer_factory()
  - enable_di(), get_di_strategy()

- Update tests to remove factory-related test cases
- Update documentation to remove factory references

This follows Kent Beck's "Fewest Elements" principle by removing
factory patterns that added complexity without providing value.

The following factories are retained and actively used:
- filter_factory: log filtering with builder pattern
- formatter_factory: output formatters with presets
- writer_factory: writer creation with presets

Closes #274

* fix(build): remove remaining DI container references from header files

- Remove di/di_container_interface.h include from logger_builder.h
- Remove DI Container examples from INTEGRATION.md documentation
- Remove DI Container reference from STRUCTURE.md table
- Remove unused mock_di_container.hpp test file

This fixes the CI build failure caused by missing di_container_interface.h.

* fix(build): resolve source_location namespace and remove outdated tests

- Fix ci::source_location to kcenon::common::source_location in
  di_pattern_example.cpp
- Remove outdated tests/unit/integration_test directory that used
  old source structure and referenced deleted files
- Update tests/unit/CMakeLists.txt to remove integration_test subdirectory