fix: add doc.go files to mock packages to fix import errors

[Kubudak90]

Description

Fixes #146

This PR adds doc.go files to the mocks/client and mocks/services packages to ensure they are properly recognized as Go packages. This resolves the 'no required module provides package' errors when running tests.

Changes

• Added mocks/client/doc.go with package documentation
• Added mocks/services/doc.go with package documentation

Problem

When running go test ./..., the following errors occur:

client/client_test.go:20:2: no required module provides package github.com/coinbase/rosetta-geth-sdk/mocks/client
client/client_test.go:21:2: no required module provides package github.com/coinbase/rosetta-geth-sdk/mocks/services

Solution

Go packages that contain only generated/mock files may not be properly recognized by the Go toolchain. Adding a doc.go file with proper package declaration ensures the packages are discoverable.

Testing

After this change, go test ./... should run without import errors.

[cb-heimdall]

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