Datasource resources API returns 500 instead of 405 for HTTP method mismatch

## Description The datasource resources API (`/api/datasources/uid/{uid}/resources/...`) returns HTTP 500 when it receives a request with an HTTP method that doesn't match the datasource's configured `httpMethod`. It should return 405 (Method Not Allowed) instead. This matters because the `prometheus/client_golang` library's `DoGetFallback` method sends POST first and falls back to GET on 405 or 501 — but the 500 response prevents the fallback from triggering, causing all queries to fail for GET-configured datasources when accessed through the resources API. ## Steps to Reproduce 1. Configure a Prometheus datasource with `jsonData.httpMethod: "GET"` 2. Send a POST request to `/api/datasources/uid/{uid}/resources/api/v1/query` with standard Prometheus query parameters 3. Observe: 500 Internal Server Error **Expected:** 405 Method Not Allowed (or ideally, the resources API should translate the incoming method to match the datasource's configured method) **Actual:** 500 Internal Server Error with `{"error":"Server Error","message":"Internal Server Error - please inspect Grafana server log for details"}` ## Additional Context - Sending a GET request to the same endpoint works correctly - The same datasource works via `/api/ds/query` (unified query API) and `/api/datasources/proxy/{id}/` (proxy API) with either method - The issue specifically affects the resources API path, which is used by the [Grafana MCP server](https://github.com/grafana/mcp-grafana) - Related upstream issue: https://github.com/grafana/mcp-grafana/issues/632 ## Impact Any tooling that uses the datasource resources API with the standard `prometheus/client_golang` library will fail for GET-configured datasources. This currently affects the Grafana MCP server's `query_prometheus` tool.