Support Grafana advanced Expressions (Math / Reduce / Resample / Classic condition)

[miguel-sanchez-elastic]

Problem

Grafana alerts have two shapes:

• With Advanced options off, the rule is a single "WHEN [reducer] OF query IS ABOVE N" — one reducer + one threshold. This is the common case and migrates today.
• With Advanced options on, the Expressions section lets users chain steps — Math, additional Reduce steps, Resample, Classic condition — and the migration parks any such alert for manual review and does not create it.

Impact

Hand-built Grafana alerts that use Advanced options (Math, Classic condition, multi-step chains) don't migrate and must be rebuilt by hand.

Steps to reproduce

1. Create an alert, enable Advanced options, and add a Classic condition (or a Math step).
2. Migrate it.

Current behavior: parked for review, not created.

Scope of translation

• Math (arithmetic/logic across steps) → expressible as arithmetic in the query. Translatable.
• Classic condition (legacy reduce + threshold in one) → expressible as aggregate + threshold. Translatable.
• Reduce variants beyond the simple form → aggregation in the query. Translatable.
• Resample (re-align a series to a fixed step) → no Kibana equivalent. Not translatable — flag and warn.

Acceptance criteria

• Math and Classic-condition alerts migrate to a working rule.
• Multi-step Reduce chains migrate where the operations have query equivalents.
• Resample (and any other untranslatable step) is clearly flagged with a reason instead of being silently parked.