Add new changes for Sync26 (geohashlist)

[albertoramosmonagas]

What type of PR is this?

Add one of the following kinds:

• enhancement/feature

What this PR does / why we need it:

This PR extends the Population Density Data API with support for a new optional area type: GEOHASHLIST. POLYGON remains the mandatory area type for all MNO implementations, while GEOHASHLIST is introduced as an optional area representation for providers that can support geohash-based requests. Changes included:

• Added new GEOHASHLIST value to areaType.
• Added GeohashList and Geohash schemas.
• Added geohash validation pattern: ^[0-9bcdefghjkmnpqrstuvwxyz]{1,12}$
• Restricted the precision field to POLYGON requests only. If precision is provided together with GEOHASHLIST, the API returns 400 INVALID_ARGUMENT.
• Added new 422 error:POPULATION_DENSITY_DATA.UNSUPPORTED_AREA_TYPE to indicate that the requested areaType is valid in the API specification but not supported by the MNO.
• Refactored CellPopulationDensityData.geohash to reference the common Geohash schema, ensuring the same geohash definition is reused in both request and response models.
• Updated the API description to clarify the expected GEOHASHLIST behaviour:
  • geohashes may have mixed precision levels;
  • geohashes do not need to be contiguous;
  • geohashes outside MNO coverage return NO_DATA;
  • if the full geohash list is outside the supported area, the response status is AREA_NOT_SUPPORTED.

This PR also clarifies the behaviour for unsupported precision values:

• If the precision value or geohash length is outside the schema range 1–12, the request is rejected with 400 INVALID_ARGUMENT.
• If the value is within the valid schema range but not supported by the MNO, the API returns:422 POPULATION_DENSITY_DATA.UNSUPPORTED_PRECISION

This applies both to the precision field for POLYGON requests and to the length of each geohash in GEOHASHLIST requests.

Which issue(s) this PR fixes:

Fixes #110, #109, #105

Special notes for reviewers:

This PR is based on conversations that can be followed at #110.

Changelog input

 release-note
Added optional `GEOHASHLIST` area support with reusable geohash schemas, clarified precision handling, and introduced `POPULATION_DENSITY_DATA.UNSUPPORTED_AREA_TYPE` for valid but unsupported area types.

Additional documentation

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