chore: simplify pagination iterator state and query-param parsing

[OmarAlJarrah]

Three small, behavior-preserving cleanups in the pagination package, grouped because they sit in the same subsystem. No public signatures move and the existing pagination tests stay green.

Paginator — drop the started flag

The PaginatorIterator carried a started boolean solely to pick initialRequest on the first fetch. Seeding nextRequest with initialRequest lets the first fetch read nextRequest exactly like every later page, so advance()'s two-stage request selection collapses into a single guard. The nextRequest ?: finishedPage.nextPageRequest() fallback was dead: by the time advance() re-runs, nextRequest already holds that page's nextPageRequest() (or null, which the page's own hasNext check short-circuits).

The page-budget increment (pagesFetched++) and the nextRequest reassignment both remain after the fetch/parse. That ordering is load-bearing: the sync iterator supports retry — if execute() throws, a caller can catch it and call hasNext() again, re-entering advance() to re-attempt the same request. Mutating either field before the fetch would burn the page budget or advance the cursor on a failed exchange.

RequestRebuilder.getQueryParam — use substringBefore/substringAfter

Replaced the hand-rolled indexOf('=') plus two conditional substring calls with substringBefore('=', segment) / substringAfter('=', ""), matching the idiom its sibling setQueryParam already uses for key extraction. The default-argument forms split on the first = and fall back to the supplied default when absent, so the no-=, empty-value, and =-in-value cases are identical.

LinkHeaderPaginationStrategy — drop the dead Link-header guards

The two empty-string guards around the header join were unreachable: an empty values(...) list joins to "", and extractNextUrl("") already returns null (an empty header splits to no link-values). The intermediate vals and branches fold into one extractNextUrl(...) call; the explicit separator = "," is kept so the join uses a bare comma.

Build

No public-API change — every edit is internal (PaginatorIterator is private, RequestRebuilder is an internal object, and parse(...) keeps its overriding signature), so apiCheck needs no apiDump. :sdk-core ktlint and the full org.dexpace.sdk.core.pagination.* test suite pass locally.

Closes #177.