Support Cloud Serverless InfluxQL through the v1-compatible /query API

## Problem

`query_influxql` is advertised for Cloud Serverless, but `QueryService.executeInfluxqlQuery()` falls through to an unsupported-product error for that product type.

Cloud Serverless supports InfluxQL through its v1-compatible `/query` endpoint, rather than `/api/v3/query_influxql`. The endpoint requires an existing DBRP mapping for the requested database/bucket.

## Expected behavior

Route Cloud Serverless `query_influxql` requests through the documented v1-compatible `/query` API, preserving the product's Token authentication and surfacing DBRP-mapping failures as structured MCP errors.

## Acceptance criteria

- Cloud Serverless `query_influxql` uses the v1 `/query` API with database and query parameters.
- Read-only query validation and bounded-result behavior are retained.
- A missing DBRP mapping returns a clear structured error with a remediation hint.
- Product-routing tests cover successful dispatch and the DBRP-mapping failure path.

## References

- [Use InfluxQL and the v1 HTTP query API](https://docs.influxdata.com/influxdb3/cloud-serverless/query-data/execute-queries/v1-http/)
- [Cloud Serverless v1 API compatibility and DBRP mappings](https://docs.influxdata.com/influxdb3/cloud-serverless/guides/api-compatibility/v1/)

Start at QueryService.executeInfluxqlQuery() and review the Cloud Serverless v1 HTTP query API references linked in the issue. Trace the existing read-only validation and bounded-result behavior, then inspect the product-routing tests; done means successful v1 /query dispatch and a structured DBRP-mapping error with a remediation hint are covered.

Written by the indexing model from the issue text.