[fix] validate sql reaching the alert query executor

[Duansg]

What's changed?

Apply read-only validation at the point where SQL reaches the executor, and fix the validator's handling of multiple statements.

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[zqr10159]

This is useful executor-level read-only validation, but it does not close the complete alert-query exposure. Low-privilege callers can still issue broad telemetry reads, expensive SELECTs, and raw PromQL; SELECT-only parsing also permits unions/subqueries across available tables. Please add an authorization/ownership boundary for preview and authoring, scope readable sources/tables, and enforce time/range/cost limits for both SQL and PromQL. I would treat the current patch as one defense-in-depth layer, not a complete remediation.