Optimizer: remove EXACT candidate; error on unservable items

[milindsrivastava1997]

Change

Remove the EXACT fallback from the optimizer's candidate set. Every item must be served by a deployed config.

• If an item has no eligible candidate (capability, window, accuracy, latency), the planner errors before solving, listing each unservable (AQE, T, accuracy_sla, latency_sla) item and why.
• Multi-statistic AQEs (avg) are handled by rewriting to sum/count (planner/optimizer: decide support for multi-statistic AQEs (e.g. avg) #419), not by EXACT.
• Remove QueryMethod::Exact, exact_query_cpu_secs, and related translator/solution paths once nothing uses them.

Part of #753.

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