feat(async): implement remaining core operation and collection stubs

[WilliamK112]

The five async core methods reported in #151 now work instead of raising NotImplementedError. Activity search returns a typed list; move accepts a body-less 204 success; bulk update returns typed per-item errors/warnings and preserves explicit nulls while omitting unset fields. The two collection helpers gather typed activity/property pages, preserve filters and caller-owned field lists, advance offsets by actual returned counts, propagate later-page errors, and stop malformed empty-page/hasMore responses with a typed error. Default activity dates are evaluated at call time.

Closes #151.

README documents the contracts and collection memory behavior. The endpoint reference was regenerated, marking search/move/bulk update as available in both clients; the collection helpers reuse existing endpoints. Nullable date annotations are corrected.

Validation: 499 tests pass, 8 skip; all 52 new transport/paging cases failed before implementation. Cases cover filtered search, 204 moves, explicit-null/unset bulk data, partial bulk failures, typed HTTP/network errors, multi-page offsets, runtime defaults, invalid inputs, empty pages, and failures after partial collection. Ruff check/format, core-module Pyright (zero diagnostics), and diff checks pass. Contracts were checked against Oracle's search, move, and bulk update references. No live tenant operations were run.