fix(plugins): complete BigQuery Agent Analytics privacy and shutdown hardening

Harden BigQueryAgentAnalyticsPlugin so it fails closed around formatter and
parser output, redacts sensitive mappings, diagnostic text, signed URIs, and
raw prompt text before inline or GCS storage, validates existing table schema
type and mode recursively at startup, and reports shutdown success only after
all owned work, clients, and executors are drained or closed. Queue accounting
is O(1), JSON nesting is bounded consistently across Python versions, and remote
drains are created and finalized on their owning event loop.

Co-authored-by: Haiyuan Cao <haiyuan@google.com>
PiperOrigin-RevId: 954858186

Author: haiyuan-eng-google