feat: implement env_nested_delimiter support for nested configuration

[FatJun]

Summary

env_nested_delimiter was declared in SettingsConfigDict but never used. This implements the actual nested env var unfolding so that variables like DATABASE__HOST=localhost are resolved into nested struct fields.

Changes

• Change env_nested_delimiter default from "__" to None (opt-in)
• Add env_nested_max_depth config (0 = unlimited)
• Add _resolve_field_type to convert BaseSettings types to struct equivalents (handles Union, Annotated, and generic types)
• Add _collect_nested_env_values with delimiter-aware env scanning
• Add 10 tests covering nesting, prefix, depth limit, dotenv, Optional

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvement
• CI/CD improvement
• Chore/maintenance

Testing

• Tests pass locally (uv run pytest -v)
• Added new tests for this change
• Updated existing tests

Documentation

• Updated README.md (if API changed)
• Updated CHANGELOG.md (if notable change)
• Added/updated code comments
• Added/updated docstrings

Checklist

• Code formatted (uv run ruff format)
• Lint checks pass (uv run ruff check)
• Tests pass (uv run pytest -v)
• Version NOT changed (maintainers bump version after merge)
• Commit messages follow conventional commits
• No breaking changes (or documented above)
• Added tests for new features
• Documentation is up to date

Additional Notes

[vilsonrodrigues]

LGTM! Thanks for the contribution, great work implementing the nested delimiter support and adding thorough test coverage.

[vilsonrodrigues]

@FatJun approved