fix: restore issue pagination tests and go-github v57 compatibility

[lin20070906]

Fixes #1

/claim #1

Summary

• Replace the invalid Issues.ListByRepository call with the go-github v57 Issues.ListByRepo API.
• Fix the local test servers that referenced server.URL before server was initialized.
• Add the missing go.sum and indirect module entries so the repository builds and tests from a clean clone.

Verification

go test ./...

Result:

?   	github.com/alisteuber4ee1/API-Integration-Pagination	[no test files]
ok  	github.com/alisteuber4ee1/API-Integration-Pagination/pkg/github	(cached)

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