test(waterdata): Split chunker tests into tests/waterdata_chunking_test.py

The chunker section had grown to 30 tests + a fake-builder harness
and shared ``_quota_response`` helper — a self-contained subsystem
sharing a file with live-API getter tests that have nothing in
common with it. The live-API file carries a ``pytest.mark.flaky``
rerun marker tuned for transient upstream errors; the chunker tests
are pure unit tests against a fake builder and shouldn't be subject
to that retry logic at all.

- Move the 30 chunker tests, ``_FakeReq`` / ``_fake_build`` /
  ``_quota_response`` helpers, and the section comment that
  introduces the fake-builder model into the new file.
- The new file imports only what it needs (``_chunking``, the
  chunking exports, and ``_construct_api_requests`` for the one
  real-builder URL-construction test).
- Drop now-unused imports from ``waterdata_test.py``
  (``itertools``, ``math``, ``quote_plus``).
- No content changes to the tests themselves; this is a relocation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thodson-usgs