refactor(errors): namespace provider error codes in gatewayErrorMap

[MargaretKerubo]

Summary

This PR restructures the gatewayErrorMap into a nested map (map[string]map[string]string) to prevent collisions between different payment providers that might reuse the same raw error codes.

It fulfills the requirements of Issue #63 by:

• Grouping M-Pesa error codes under an "mpesa" namespace.
• Creating a "default" namespace for the generic error code examples.
• Updating the MapGatewayErrorCode function signature to accept a provider argument, allowing it to search the provider-specific map before falling back to the "default" map.

Type of change

• Bug fix
• New feature
• Refactor
• Documentation

Related issue

Closes #63

Details

• Changed gatewayErrorMap type to map[string]map[string]string.
• Updated MapGatewayErrorCode(provider, code string) string to check the provider namespace first, then fall back to "default".
• Verified that there are no current usages of MapGatewayErrorCode in the provider packages that need updating (this lays the groundwork for mapping M-Pesa endpoint responses in Issue Map M-Pesa endpoint responses onto shared paykit.Response #59).

Testing

• Ran mise run ci locally. Verified that the code compiles, golangci-lint passes without warnings, and all existing tests pass (go test ./...).

Checklist

• Tests added or updated
• Documentation updated
• go test ./... passes
• Breaking changes documented