Power BI storage reports: incremental refresh isn't detectable out of the box

🤖 [AI] Filed from findings raised in discussion #1799.

⚠️ Problem

Configuring Power BI Incremental refresh against the toolkit's storage-based reports (e.g. Cost summary) doesn't work — the Incremental Refresh dialog can't detect RangeStart/RangeEnd as assignable to any column. Reported independently by two users.

🛠️ Solution

Root cause, verified against src/power-bi/storage/Shared.Dataset/definition/expressions.tmdl, tables/Costs.tmdl, and src/power-bi/queries/ftk_NormalizeSchema.pq:

1. RangeStart/RangeEnd are only used inside the shared ftk_Storage function to pre-filter which parquet files are read — they're never applied as a Table.SelectRows step directly on the top-level Costs query, which is what Power BI's Incremental Refresh dialog needs to detect the feature.
2. ChargePeriodStart is cast to type date (not datetime) in ftk_NormalizeSchema.pq, which doesn't match the Date/Time-typed RangeStart/RangeEnd parameters.

Fix: cast ChargePeriodStart to type datetime, and add a Table.SelectRows step filtering on RangeStart/RangeEnd as the last step of the Costs query in the Shared.Dataset definition, so Power BI can auto-detect and apply incremental refresh.

ℹ️ Additional context

A manual workaround was posted in the discussion in the meantime. Also worth a docs update to docs-mslearn/toolkit/power-bi/setup.md once fixed.

Related: #1799 (discussion)

Start with src/power-bi/storage/Shared.Dataset/definition/expressions.tmdl, tables/Costs.tmdl, and src/power-bi/queries/ftk_NormalizeSchema.pq to trace the existing storage query and parameter types. Verify the change through Power BI's Incremental Refresh dialog, and review docs-mslearn/toolkit/power-bi/setup.md for the documented workaround. Done means Power BI detects RangeStart and RangeEnd for the Costs query and the setup documentation is current.