fix(admin): paginate GitHub organization listing in GetGithubUserStatus

[Tyagiquamar]

Fixes #5284.

GetGithubUserStatus called Organizations.List with nil options, so only the first page (default 30 entries) of organizations was returned. Users belonging to many organizations got a truncated org list and incomplete OrganizationInstallationPermissions.

Uses a paginated listOrganizations helper (PerPage 100, follows NextPage), matching the existing pattern in fetchReposForUser/fetchReposForInstallation.

Verified in golang:1.26.5-bookworm (matches go.mod):

• gofmt -l admin/server/ clean
• go vet ./admin/server/ pass
• go build ./admin/... pass
• go test ./admin/server/ -run TestListOrganizationsPagination pass (new test, 2-page httptest mock covering both the authenticated-user and named-user routes; fails against the old single-call logic which returns only page one)
• existing TestMirrorGitRepo still passes

[k-anshul]

Thanks for the contribution @Tyagiquamar
I made some trivial go guidelines related refactor. LGTM.