Improve tariff models: seasonal months list, optional fixed charge, d…

[AadilLatif]

Change Summary

SeasonalTOURates

• season: Month → months: List[Month] — A single entry can now cover multiple months (e.g., summer = [JUNE, JULY, AUGUST]) instead of requiring one entry per month.
• Added duplicate-month validator within each entry.

DemandCharge

• Added months: List[Month] — Demand charges can now be scoped to specific months, enabling seasonal variation (e.g., higher summer demand rates).
• Added duplicate-month validator within each entry.

FixedCharge

• amount constraint relaxed: gt=0 → ge=0 — allows $0 fixed charges.

DistributionTariff

• fixed_charge is now optional: FixedCharge (required) → Optional[FixedCharge] defaulting to None.
• Added overlapping-month validator across seasonal_tou entries — ensures no month appears in more than one SeasonalTOURates.

Related issue number

Checklist

• The pull request title is a good summary of the changes.
• Tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including "please review" to assign reviewers