Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@src/lampyrid/models/lampyrid_models.py`:
- Around line 790-791: The model currently substitutes silent defaults for
required fields by using budget_id=attrs.budget_id or '' and
amount=float(attrs.amount) if attrs.amount is not None else 0.0; update the
model to preserve None instead of masking missing data by changing the field
type annotations to Optional[str] and Optional[float] and remove the fallback
substitutions in the factory/constructor code that consumes attrs (leave
budget_id as attrs.budget_id and set amount to float(attrs.amount) only when
attrs.amount is not None, otherwise None); ensure the class-level type hints and
the classmethod that builds instances from attrs are updated together to reflect
Optional types.

In `@tests/integration/test_budgets.py`:
- Around line 469-489: The test test_delete_budget_limit creates a budget limit
via mcp_client.call_tool('set_budget_limit') but doesn't register it with the
budget_limit_cleanup fixture, so a failed delete/assert can leak state; after
the set_budget_limit call (the created limit), register that created limit with
the budget_limit_cleanup fixture (e.g., call budget_limit_cleanup.register or
the fixture's add method with the created limit id/period) so the fixture will
remove the limit on teardown if the in-test deletion fails.

---

Nitpick comments:
In `@src/lampyrid/services/budgets.py`:
- Around line 281-287: _resolve_period currently defaults to the current month
when one of start_date/end_date is provided and the other is None; update it to
raise a ValueError when exactly one of start_date or end_date is non-None to
enforce the same validation already present in the request models
(SetBudgetLimitRequest, DeleteBudgetLimitRequest via _BudgetLimitRequestBase).
Locate the _resolve_period function and replace the silent fallback branch with
an explicit check that raises ValueError('Provide both start_date and end_date,
or neither.') when (start_date is None) != (end_date is None), otherwise keep
the existing behavior for both provided or both None.