fix(mem_scheduler): tolerate absent optional fields in EnvConfigMixin.from_env (#2424)

[RerankerGuo]

Summary

EnvConfigMixin.from_env raised a bare raise ValueError() (empty message) whenever a field declared as field: int | None = None had no environment variable set — so any config with a plain optional field could not be built from the environment at all. The scheduler init path (memos/configs/mem_scheduler.py calls RabbitMQConfig.from_env()) was the most visible victim: port: int | None is exactly that shape, so it failed immediately.

Root cause

for field_name, field_info in cls.model_fields.items():
    ...
    if env_var in os.environ:
        field_values[field_name] = cls._parse_env_value(...)
    elif field_info.default is not None:
        field_values[field_name] = field_info.default
    else:
        raise ValueError()                       # ← empty, swallows any Optional[None]

field_info.default is None for Optional[int] = None, so the elif never fires and the bare raise ValueError() triggers for every absent optional field. The error also makes it impossible to tell which field caused the failure.

_parse_env_value had a secondary issue: it only parsed when target_type is bool/int/float, so an Optional[int] env var that was provided would have been left as a raw string instead of being cast.

Fix

• from_env now skips fields with no env var entirely; only genuinely required fields (no default and not Optional) still raise, and the raised error carries the missing env var name and the word "missing" (replacing the empty-message bug).
• _parse_env_value unwraps Optional[X] / X | None via typing.get_args, so an Optional[int] env var parses as int.
• A new module-level helper _is_optional keeps the Optional check in one place, used by both from_env and _parse_env_value.

Tests

New tests/mem_scheduler/test_env_config_mixin.py, 5 cases:

1. test_absent_optional_int_field_falls_back_to_none — the bug reproducer (red against unpatched main)
2. test_absent_optional_str_field_falls_back_to_default — same bug for non-Optional defaults
3. test_optional_int_env_var_parses_as_int — secondary symptom (Optional[int] gets cast)
4. test_bool_env_var_parses_as_bool — regression coverage
5. test_missing_required_field_raises_named_value_error — genuinely required fields still raise, with a meaningful message

Verification

PYTHONPATH=src pytest tests/mem_scheduler/test_env_config_mixin.py -v
============================== 5 passed in 2.12s ==============================

Ruff: ruff format --check clean on touched files; ruff check 0 new findings vs main (5 pre-existing BLE001/B008/UP findings auto-fixed by ruff check --fix during this PR, 1 pre-existing TC009 finding remains, both unrelated to this change).

Out of scope

• No new dependencies
• No public API / OpenAPI contract changes
• No DB schema, migration, or graph model changes
• No Makefile / pyproject.toml / pre-commit / GitHub workflow changes

Closes #2424