Handle nested overrides that collide with non-mapping values

[Imkkey]

Fixes #215.

EnvSource(parse_yaml_docs=True) crashes when a nested environment variable overrides a leaf already present in a parent YAML mapping. The same collision affects nested dictionaries combined with dotted set_args keys.

For example, APP_DATABASE='{host: localhost, port: 5432}' and APP_DATABASE__PORT='5433' currently call 5432.update(5433).

Only merge when both leaf values are dictionaries; otherwise retain the more-specific value. This follows the existing parent-before-child ordering. Existing shallow mapping merges, key order, and set_args handling of omitted/None arguments remain unchanged.

Tests cover scalar, list, null and mapping collisions, both insertion orders, dictionary and Namespace arguments, preserved source inputs, and shallow mapping merge/order. Add an Unreleased changelog entry.

Validation on Python 3.12.14:

• Before: 108 new regression cases fail; 25 new controls pass.
• After: all 133 new cases and all 432 core tests pass.
• Full suite: 433 passed, one pre-existing failure in test_bump_version_applies_sequential_changelog_updates. Clean baseline: 300 passed with the same failure, caused by the fixed September-length underline for an October date.
• Ruff lint/format, full mypy with the CI Python 3.10 target, strict Sphinx HTML build, changelog formatting/lint, and git diff --check pass.

No other Python runtime or remote CI has been tested for this patch. Prepared and checked with OpenAI Codex assistance; draft for maintainer review.