fix(go): Refactor mappers to own module

[devsjc]

Moves all mapping logic to it's own file, simplifying the logic in the RPC implementations. Also enables testing of the mapping outside of testing business logic.

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• Have you followed the Open Climate Fix Contribution Guidelines?
• Have you referenced the Issue this PR addresses, where applicable?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added a summary of the changes?
• Have you written new tests for your changes, where applicable?
• Have you successfully run make lint with your changes locally?
• Have you successfully run make test with your changes locally?

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[github-actions]

Benchmark Results

Benchmark results

?   	github.com/openclimatefix/data-platform/cmd	[no test files]
?   	github.com/openclimatefix/data-platform/internal/gen/ocf/dp	[no test files]
?   	github.com/openclimatefix/data-platform/internal/interceptors	[no test files]
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/dummy	0.005s
{"level":"debug","time":"2026-07-29T09:42:01Z","message":"Completed migrations"}
goos: linux
goarch: amd64
pkg: github.com/openclimatefix/data-platform/internal/server/postgres
cpu: AMD EPYC 7763 64-Core Processor                
BenchmarkPostgresClient/small/GetForecastAsTimeseries-4         	      50	  22904431 ns/op
BenchmarkPostgresClient/small/GetForecastAtTimestamp-4          	     180	   6545414 ns/op
BenchmarkPostgresClient/small/GetObservationsAsTimeseries-4     	     930	   1246732 ns/op
BenchmarkPostgresClient/small/CreateForecast-4                  	     105	  10990233 ns/op
BenchmarkPostgresClient/small/StreamForecastData-4              	      21	  52534935 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	68.665s
?   	github.com/openclimatefix/data-platform/internal/server/postgres/gen	[no test files]