fix(plugins): harden BigQuery agent analytics against fail-open privacy, GCS concurrency, and startup-loss gaps

Close several correctness and privacy gaps in the BigQuery agent analytics
plugin:

- content_formatter failures fail closed with a [FORMATTER_FAILED] sentinel
  instead of falling back to the unformatted payload; the exception is logged
  without the payload and counted as formatter_failed in get_drop_stats().
- A final sanitizer pass runs over the fully assembled attributes tree before
  json.dumps, covering state deltas, custom tags, labels, nested structures,
  and temp:-scoped keys. A new _sanitize_json_blob redacts sensitive keys
  inside JSON-encoded string blobs (e.g. cached credential JSON): it decodes
  container-shaped strings first, enforces max_content_length before
  json.loads, and fails closed to [UNPARSEABLE_JSON_BLOB] for anything it
  cannot parse and verify.
- GCS offload paths are call-local: parse() and _parse_content_object() take
  trace_id/span_id as keyword arguments and build every object path from
  those values, so concurrent two-part offloads never collide on an object
  name.
- Table readiness becomes a startup requirement with bounded retry:
  _ensure_schema_exists raises on failure, and _ensure_started records it,
  keeps _started False, and retries on a later event after exponential
  backoff (2s to 60s cap). Setup is coalesced across event loops and threads,
  so a persistent outage costs one setup RPC per backoff window. Rows
  arriving while setup is unavailable are counted as setup_unavailable.

Also: enabled=False is a hard no-op with zero side effects, runtime settings
are validated at construction, and plugin-level drop counters survive
shutdown.

Co-authored-by: Haiyuan Cao <haiyuan@google.com>
PiperOrigin-RevId: 949235728

Author: haiyuan-eng-google