bug: legacy soil‑depth definition does not handle custom intervals

[jjmaynard]

Description

The SoilID API’s legacy depth‐handling logic only requires a bottom_depth (e.g. 1 for the 0–1 cm interval, 10 for the 1–10 cm interval, …) and infers the top_depth automatically. The new LandPKS app offers three soil depth interval options—legacy (BLM), NRCS/FAO standard, or custom intervals—but the API still expects only bottom depth ('horizonDepth'). As a result, custom intervals can’t be processed correctly because their top_depth cannot be correctly derived.

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Steps to Reproduce

1. In the LandPKS app, select Custom depth intervals.
2. Define a set of custom intervals, for example:

   [
     { "bottom_depth": 5 },
     { "bottom_depth": 20 }
   ]

3. Submit these intervals in a call to the SoilID API endpoint (e.g. POST /rank_soils).
4. Observe that the API either:
   • Returns a malformed result (missing or incorrect interval boundaries), or
   • Silently ignores the custom depths and falls back to legacy defaults.

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Expected behavior

• The API should require both top_depth and bottom_depth for all depth intervals.
• If either boundary is missing for an interval, the API should return a clear validation error, e.g.:

  HTTP/1.1 400 Bad Request
  {
    "error": "`top_depth` and `bottom_depth` are required for custom intervals"
  }

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Actual behavior

• Custom intervals sent with only bottom_depth are processed using legacy defaults, leading to incorrect or missing soil‐ranking results.
• No validation error is returned to inform the client that top_depth is missing.

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Additional Notes

*It is unclear to me whether this issue has already been addressed in the production code currently used to support the new app.