fix(forecast): validate query field parameter and reject invalid values with 400 (closes #891)

[mopxam-art]

Overview

Closes #891 by validating the field query parameter across forecast routes (/:id, /seasonal/:id, and /accuracy/:id) and rejecting unknown metrics with 400 Bad Request listing valid options (power_output_kw, efficiency_pct), eliminating silent data substitution.

Problem & Motivation

Previously, parseForecastField defaulted any string other than "efficiency_pct" to "power_output_kw". A typo such as ?field=efficiency or ?field=power was silently coerced to power_output_kw, returning 200 OK with unexpected data. Sibling endpoints uniformly reject unrecognized query enums with 400 Bad Request.

Changes

• src/routes/forecast.ts:
  • Defined VALID_FORECAST_FIELDS = ["power_output_kw", "efficiency_pct"] as const.
  • In parseForecastField, preserved default "power_output_kw" for omitted or empty field parameters.
  • Throws badRequest when an unknown metric name is supplied, explicitly listing valid options in the error message.
• src/__tests__/forecastFieldValidation.test.ts:
  • Added 110 exhaustive unit tests covering boundary clamping, typo combinatorics, URL encodings, format variations (JSON/CSV), and state isolation.

Verification

• All 155 unit tests pass cleanly (npx jest src/__tests__/forecast.test.ts src/__tests__/forecastFieldValidation.test.ts --forceExit).
• TypeScript typecheck passes with 0 errors (npx tsc --noEmit).
• ESLint passes with 0 warnings/errors (npx eslint src/routes/forecast.ts).
• Production build compiles cleanly (npm run build).