Refactor client to consistently use GraphQL API. Refactor tests

[chrisreddington]

This pull request includes significant changes to the github/client.go and main_test.go files to transition from using REST API calls to GraphQL queries. Additionally, it refactors the tests to use the new MockGitHubClient from the mocks package, reducing some duplication across both of those files.

Throughout this PR, I'll also aim to do further cleanup in separating out those concerns.

Copilot reviewed 5 out of 8 changed files in this pull request and generated 1 comment.

Files not reviewed (3)

• github/client.go: Evaluated as low risk
• github/client_test.go: Evaluated as low risk
• main.go: Evaluated as low risk

Comments suppressed due to low confidence (2)

types/types.go:42

• The removal of the 'Data' field from the ContributionsResponse struct changes the expected structure of the API response. Ensure that all parts of the code that parse this response are updated accordingly to avoid runtime errors.

Login                   string `json:"login"`

main_test.go:22

• The MockBrowser should return m.Err only if m.Err is not nil to avoid returning a nil error when no error is expected.

return m.Err

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The FetchContributions method should return the MockData field if it is set, rather than always generating new mock data.

Suggested change

	return fixtures.GenerateContributionsResponse(username, year), nil
	// Return MockData if it is set, otherwise generate new mock data
	if m.MockData != nil {
	return m.MockData, nil
	}
	// Always return generated mock data with valid contributions