PvMetadataClient query ignores limit=0 (truthiness bug)

[craigmcchesney]

PvMetadataClient._build_query_pv_metadata_request() sets the request limit with a truthiness check:

if limit:
    request.limit = limit

This drops a caller-supplied limit=0, so a query intended to request zero (or the server-default) page size silently omits the field. The correct guard is is not None:

if limit is not None:
    request.limit = limit

The newer MachineConfigClient query builders already do this correctly (with a test_build_query_request_limit_zero_is_set regression test) — this was noted as a cross-PR follow-up during the #9 code review (PR #12). This issue tracks back-porting the same fix (and an equivalent limit=0 test) to pv_metadata_client.py for consistency.

Fix location: src/dp_python_lib/client/pv_metadata_client.py — _build_query_pv_metadata_request.