geolocation: add Go Read SDK for GeolocationUser types and client methods

[ben-dz]

Resolves: #3012

Summary of Changes

• Add GeolocationUser struct and related types (GeolocationTarget, GeolocationBillingConfig, enums) with Borsh serialization/deserialization matching the Rust onchain layout
• Add DeriveGeolocationUserPDA() using seeds ["doublezero", "geouser", code]
• Add GetGeolocationUserByCode() and GetGeolocationUsers() client read methods
• Add AccountTypeGeolocationUser = 3 discriminator, GeolocationUserSeed, and MaxTargets constants

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	4	+351 / -3	+348
Tests	4	+405 / -0	+405

Core logic and tests in roughly equal proportion.

Key files (click to expand)

• sdk/geolocation/go/state.go — new types (GeolocationUser, GeolocationTarget, GeolocationBillingConfig, enums) with Serialize/Deserialize methods
• sdk/geolocation/go/client.go — GetGeolocationUserByCode and GetGeolocationUsers methods
• sdk/geolocation/go/pda.go — DeriveGeolocationUserPDA function
• sdk/geolocation/go/deserialize.go — DeserializeGeolocationUser discriminator wrapper
• sdk/geolocation/go/constants.go — GeolocationUserSeed and MaxTargets constants

Testing Verification

• Serialization round-trip tests for GeolocationUser with targets, empty targets, and individual target structs
• Deserialize wrapper tests: happy path, wrong account type, truncated data
• PDA derivation tests: determinism, uniqueness across codes, empty/too-long code rejection
• Client tests: mock RPC happy path and not-found for both single and batch fetch
• Enum String() method coverage for all new enum types