Add Argentine

[thiagovmdon]

Summary

Related to issue #73

This PR rewrites the Argentina fetcher to follow INA Alerta's live a5 API logic instead of the previous simplified endpoint approach.

What changed

• Reworked ArgentinaFetcher to use:
  • obs/puntual/series to resolve discharge series metadata
  • obs/puntual/estaciones/{station_id} to enrich station metadata
  • getObservaciones with series_id and csvless responses for timeseries downloads
• Limited Argentina support to the verified variable (following R implementation from GRDC):
  • discharge_daily_mean
• Fixed live-download behavior for INA responses:
  • handle timezone-aware timestamps
  • treat end_date as inclusive even though INA's timeend behaves effectively as exclusive
• Standardized cached and live metadata to the RivRetrieve metadata shape
• Regenerated rivretrieve/cached_site_data/argentina_sites.csv from live metadata
• Added Argentina tests covering:
  • cached metadata normalization
  • live metadata assembly
  • csvless timeseries parsing
  • station 8 discharge retrieval flow