fix(plugins): memoise BigQuery analytics readiness across re-initialisation

[chelsealong]

Summary

Closes #7017.

BigQueryAgentAnalyticsPlugin re-ran its full table/view readiness pass on every re-initialisation instead of once per process. Each pass issues one CREATE OR REPLACE VIEW statement per _EVENT_VIEW_DEFS entry (25 at HEAD), and it is awaited from before_run_callback, so the DDL sits on the agent request path.

Any deployment that shares one plugin instance across short-lived Runners (Runner.close() → PluginManager.close() → plugin.close() clears _started) re-runs all the view DDL per request. The reporter measured ~19k view-DDL jobs/hour, per-table BigQuery quota exhausted in ~2h, and ~25s added to median latency — a 2.3.0 → 2.8.0 regression.

In 2.3.0 the readiness pass was gated behind the schema cache, so it ran at most once per process. At HEAD that gate was deliberately removed so a failed first attempt would still retry — but the comment's load-bearing assumption ("once _started is True the steady state pays no extra RPC") only holds when the plugin is initialised once per process, which is false whenever it is closed and reused.

Fix

• Memoise readiness success in a new _schema_ready flag that, like _schema, survives close()/shutdown(), and gate _ensure_schema_exists on it in _lazy_setup.
  • A failed attempt raises before the flag is set, so it is still retried on the next setup (preserves the 2.8.0 intent).
  • A successful attempt is memoised, so it is not repeated per re-initialisation (restores the 2.3.0 cost profile). No new steady-state RPC.
• Secondary: emit exactly one WARNING when a generation mismatch aborts an otherwise-successful setup, so a plugin churning through full setups is no longer completely silent (the issue observed 12 log lines against ~2000 setup runs).

Scope is limited to the readiness memoisation and the one diagnostic log line. The view SQL, _EVENT_VIEW_DEFS, and the _ensure_started backoff are untouched (the issue's non-goals).

Testing Plan

Added TestReadinessMemoisedAcrossReinit with two tests:

• test_readiness_pass_runs_once_across_reinit_cycles — 3 _ensure_started() / shutdown() cycles over one shared plugin issue the view DDL once total (client.query called len(_EVENT_VIEW_DEFS) times), not once per cycle.
• test_failed_readiness_is_retried_and_not_memoised — a first readiness attempt that raises leaves _schema_ready False and is retried on the next setup, which then succeeds and memoises.

Evidence the tests prove the bug: with the source change stashed (test kept), both new tests fail (AttributeError: ... _schema_ready / would-be 75 vs 25 DDL statements); with the fix restored they pass.

# without the fix (source change stashed):
2 failed in 24.20s

# with the fix:
tests/unittests/plugins/test_bigquery_agent_analytics_plugin.py  505 passed, 6 skipped in 482.90s

Lint/format on the changed files: pyink (2 files unchanged), isort (exit 0), ruff check (all checks passed).

AI assistance disclosure

This change was prepared with AI assistance (Claude). A human reviewed the diff, the tests, and the failing-without-fix evidence before submission.