perf(bigquery): [T2-1] add bounded 1-page lookahead prefetching to TableResult.iterateAll()

[lqiu96]

Summary & Bottleneck

In java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/TableResult.java, TableResult.iterateAll() delegates to getPageNoSchema().iterateAll(), which uses PageImpl.PageIterator.

1. Synchronous page-boundary stalls:
   • PageImpl.PageIterator only calls currentPage.getNextPage() synchronously after the caller has exhausted all rows of the current page (!currentPageIterator.hasNext()).
   • While the caller processes the rows of Page $k$, no network or deserialization work occurs for Page $k+1$. When the caller reaches the end of Page $k$, iteration stalls for the full round-trip latency and deserialization time of getNextPage().

Changes

• java-bigquery/google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/TableResult.java:
  • Introduce PrefetchingPageIterator in TableResult.iterateAll() backed by a shared daemon thread pool (PREFETCH_EXECUTOR).
  • When iteration begins on a page where currentPage.hasNextPage() is true, submit currentPage.getNextPage() asynchronously so the next page is fetched and deserialized in parallel with the caller consuming the current page.
  • Bound lookahead strictly to at most 1 in-flight page per iterator, and never submit background tasks for single-page results (hasNextPage() == false) or for TableResult.getValues().
  • Preserve exact exception propagation semantics at page boundaries by unwrapping ExecutionException causes when advancing to the prefetched page.

Expected Impact

• Overlaps the network round-trip and row deserialization latency of Page $k+1$ with application processing of Page $k$ during TableResult.iterateAll() across both REST (tabledata.list) and Storage Read API (ArrowQueryPageFetcher) pagination paths.
• Keeps memory bounded to at most 1 lookahead page and adds zero thread overhead to single-page queries.

Verification

• Added unit tests in java-bigquery/google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/TableResultTest.java verifying asynchronous background prefetch during Page 0 iteration, 1-page lookahead bounding across a 3-page chain, and exception propagation at the page boundary.
• Ran mvn test -pl java-bigquery/google-cloud-bigquery -Dtest=TableResultTest,ArrowQueryPageFetcherTest,JobTest,BigQueryImplTest (191 tests run, 0 failures).