Close mdapi sub-resource gaps and fix key-name mismatches

[GClunies]

What & Why

mdapi coverage is mostly present but has known gaps and two key-name mismatches that silently break data loading for stations with specific features. Closing these makes the mdapi surface complete and removes latent bugs. Part of #89.

Acceptance Criteria

• supersededdatums is reachable as a Station attribute (or equivalent method). Test cassette exercises a station that has superseded datums.
• Harmonic constituents load correctly: either the expand param is fixed from harcon → harmonicConstituents or the response-key read at _metadata.py:123 is fixed to match what NOAA actually returns. Verified against a live API hit.
• Tide prediction offsets handle the tidepredoffets vs tidepredoffsets mismatch correctly. The expand param currently preserves NOAA's historical typo (_metadata.py:37-38) while the response-read uses the correctly spelled key (_metadata.py:137) — confirm which NOAA actually honors and fix whichever side is wrong.
• Standalone fetchers exist for the sub-resources that have their own URLs: /stations/{id}/supersededdatums.json, /details.json, /sensors.json, /datums.json, /harcon.json, /floodlevels.json. Callers should be able to refresh a single sub-resource without reconstructing a full Station.
• Existing Station() construction continues to pass all tests.

Approach

• Start by reproducing the harcon mismatch against a live mdapi response for a station with tidal constituents (e.g. 8454000 or a similar primary water-level station). Decide which side of the mismatch is wrong before editing — don't guess.
• Add a single helper (e.g. _fetch_mdapi_subresource(station_id, name)) rather than one method per sub-resource, and expose thin wrappers on Station.
• Files in play:
  • noaa_coops/_metadata.py (all of it; key-name fixes at :37-38, :123, :137).
  • noaa_coops/_endpoints.py (base URL plumbing for sub-resource fetchers).
  • tests/test_station.py + new VCR cassettes.

Resources

• Key-mismatch lines: noaa_coops/_metadata.py:37-38, :123, :137.
• mdapi base URL: https://api.tidesandcurrents.noaa.gov/mdapi/prod/webapi/stations/
• Existing populate methods for pattern reference: noaa_coops/_metadata.py:86-163.
• Parent epic: Epic: Full NOAA CO-OPS API coverage and remove SOAP metadata dependency #89.