Renames the cursor pagination request field from cursor to start_cursor
across the three V2.3 list endpoints (customers, entities, events).
Updates schemas, handlers, downstream services, OpenAPI contracts, the
test SDK client, and affected integration tests. Also fixes a latent bug
in list-customers-v22-vs-v23-parity where fetchV23 hardcoded limit:1
instead of LIMIT, causing flaky parity failures when the dev org had
more than one customer.
ef067da914