client/resource_group: expose RU v1 calculation details

## Enhancement Task

TiDB currently reports the total RU consumed by a statement, but RU v1 users cannot inspect the request inputs and effective factors that produced it. This is the PD-side work for pingcap/tidb#70251.

The PD resource controller owns the authoritative RU calculation and effective configuration. Expose an optional request-scoped calculation detail from the existing KVCalculator path containing:

- the dynamic inputs used by the RU v1 calculation;
- the effective factor snapshot;
- the actual RRU and WRU results.

Downstream consumers can format these inputs as a formula and independently compare the result with the authoritative RRU and WRU values.

Requirements:

- Keep existing ResourceGroupKVInterceptor signatures unchanged.
- Preserve token accounting, limiting behavior, and RU v2 behavior.
- Avoid per-request string formatting and collection unless explicitly requested.
- Cover read, write, retry, failed-write payback, replica weighting, and paging settlement paths.

Related work:

- TiDB issue: https://github.com/pingcap/tidb/issues/70251
- PD implementation: https://github.com/tikv/pd/pull/11165
- client-go integration: https://github.com/tikv/client-go/pull/2046
- TiDB integration: https://github.com/pingcap/tidb/pull/70543

Start by reading the existing KVCalculator path and ResourceGroupKVInterceptor, then compare the referenced PD implementation and client-go integration. Done means an optional request-scoped detail exposes the calculation inputs, effective factors, and RRU/WRU results while preserving existing signatures, accounting, limiting, RU v2 behavior, and the listed settlement paths.

Written by the indexing model from the issue text.