feat(serialize): add env/dotenv format to FileFormat and serialization layer

[EspenAlbert]

Summary

Add .env (dotenv) format support to model_lib, making it a first-class format alongside json, yaml, and toml.

• Add FileFormat.env to the FileFormat StrEnum.
• New serialize/env_serialize.py with parse_env_str (via python-dotenv's dotenv_values) and dump_env_str for flat dict output.
• Register the parser in _format_parsers and .env in _file_format_to_raw_format.
• Fix _parse_path to handle dotfiles: Path(".env").suffix is "", so the path dispatcher now falls back to payload.name when suffix is empty.
• Add python-dotenv>=1.0 as a direct dependency (already transitive via pydantic-settings).

Changes

File	What
model_lib/constants.py	Add env = "env" to FileFormat
model_lib/serialize/env_serialize.py	New: parse_env_str, dump_env_str
model_lib/serialize/parse.py	Register env parser; fix dotfile suffix detection in _parse_path
model_lib/serialize/dump.py	Register dump_env_str in _payload_dumpers
model_lib/readme_example_test.py	Add "env" to expected_dump_formats guard
model_lib/serialize/env_serialize_test.py	New: 5 tests covering parse, dump, and Path dispatch
pyproject.toml	Add python-dotenv>=1.0 to dependencies

Design decisions

• dump_env_str only accepts flat dicts: dotenv format is inherently flat (KEY=VALUE). Nested structures will raise TypeError. This is intentional — env format is load-oriented.
• # no cov on import lines: coverage.py does not record module-level import statements as hits (they execute before the measurement window closes). The existing toml imports have the same property; those pre-existing lines are not flagged because they are not in the diff.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 46.15385% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.87%. Comparing base (12cfa50) to head (c9633f6).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
model_lib/serialize/env_serialize.py	50.00%	5 Missing ⚠️
model_lib/serialize/dump.py	0.00%	1 Missing ⚠️
model_lib/serialize/parse.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   32.77%   32.87%   +0.09%     
==========================================
  Files          18       19       +1     
  Lines        1068     1080      +12     
==========================================
+ Hits          350      355       +5     
- Misses        718      725       +7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[EspenAlbert]

do not use #no cov, instead make sure it is covered by tests