feat: Update serde serialization for Ellipsoidal Position to use expl…

[VPRamon]

This pull request updates the serialization format for ellipsoidal positions to use explicit degree suffixes for longitude and latitude fields, improving clarity and consistency. The most important changes are grouped below:

Serialization format update:

• Changed the serialized field names from lon and lat to lon_deg and lat_deg in the documentation and implementation, making it clear that these values are always in degrees. (src/ellipsoidal/position_serde.rs) [1] [2]
• Updated error messages and field handling in deserialization to use lon_deg and lat_deg instead of the previous lon and lat, ensuring backward compatibility and clear error reporting. (src/ellipsoidal/position_serde.rs) [1] [2] [3]

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Note

Medium Risk
Changes the on-the-wire serialized field names for positions, which can break existing persisted data and client integrations unless migrated.

Overview
Updates ellipsoidal::Position serde to emit and require lon_deg/lat_deg field names (instead of lon/lat), with corresponding updates to documentation and deserialization error messages.

Adds a changelog entry noting the serialization breaking change for downstream consumers expecting the old JSON field names.

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[github-actions]

Package	Line Rate	Complexity	Health
src	100%	0	✔
src.cartesian	98%	0	✔
src.frames	93%	0	✔
src.spherical	94%	0	✔
src.ellipsoidal	71%	0	➖
src.ops	100%	0	✔
Summary	95% (2510 / 2646)	0	✔