Add req() guard for NULL provider/model in query builder

[seanthimons]

Source: PR #162 review (round 1, item #7)

File: R/mod_query_builder.R lines 74-75

Problem: resolve_model_for_operation() can return NULL but no req() guard exists. Produces cryptic downstream errors.

Fix: Add req(provider, model, cancelOutput = TRUE) after resolution.