Add existing pumped hydro storage units

[WeiAi-Energy]

Closes #758.

Changes proposed in this Pull Request

This PR adds existing pumped hydro storage (PHS) units from powerplants.csv to add_electricity.

• Adds prime_mover_code == "PS" rows as StorageUnit components with carrier="PHS".
• Uses CSV p_nom and build_year for existing PHS units.
• Sets PHS as non-extendable with infinite lifetime.
• Sets store/dispatch efficiencies to 0.894427191 and max_hours to 24.0 / efficiency_dispatch.
• Excludes prime_mover_code == "PS" rows from renewable hydro capacity assignment to avoid double counting as conventional hydro.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml. N/A, no dependency changes.
• Changes in configuration options are added in all of config.default.yaml. N/A, no configuration changes.
• Changes in configuration options are also documented in doc/configtables/*.csv. N/A, no configuration changes.