Add ZenRowsBatchClient — a synchronous, typed client for the async-job
Batch API — and migrate the package to a src/ layout.

- Typestate resource handles: JobRef/JobHandle, RunRef/RunHandle,
  ExportRef/ExportHandle (id-only refs vs loaded handles with .data),
  with job.run.* (current run) and job.schedule.* facets that
  disambiguate the two "pause" endpoints.
- submit_regular / submit_open / submit_scheduled, typed schedule
  builders (At/Rate/Calendar), CSV file-input upload, and cursor-free
  scanners (iter_jobs / iter_runs / iter_results).
- Waiters (run / ingest / export) with jittered backoff; transport-level
  retries of transient failures (429/502/503/504 + network errors) on
  idempotent requests, honoring Retry-After.
- Downloads pulled straight from each result's presigned result_url (no
  /content endpoint): bulk download_to_dir / download_to_memory,
  server-side zip download_all_results, and per-task
  download_task_to_file / download_task_to_memory.
- Webhook management (get/put/delete job webhook, test_webhook), HMAC key
  lifecycle, and offline cost estimation (client.estimate_cost).
- RFC 7807 error mapping to BatchAPIError; pydantic v2 models generated
  from the OpenAPI spec; generated markdown API reference and runnable
  examples.

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