Add item count warning in query editor for large result sets (#2152)

## Summary

Adds a non-intrusive warning banner in the query editor that alerts
users when their query matches a large number of items (>= 500). This
helps users understand that their query may return a large amount of
data and suggests using more specific filters.

Part of https://github.com/grafana/oss-big-tent-squad/issues/127

## Changes

- Added `ITEM_COUNT_WARNING_THRESHOLD` constant (500 items) in
`src/datasource/constants.ts`
- Created new `ItemCountWarning` component in
`src/datasource/components/ItemCountWarning.tsx`
- Updated `MetricsQueryEditor` to track and report the count of items
matching the current filter
- Integrated the warning component into the main `QueryEditor` component

## How it works

- When items are loaded for the dropdown in the Metrics query editor,
the component counts how many items match the current item filter
- If using a regex filter like `/.*/`, it applies the regex to count
matching items
- If the count is >= 500, a warning banner appears at the top of the
query editor
- The warning is purely informational - queries still execute normally
- The warning only appears for the "Metrics" query type

## Screenshot

The warning appears as a subtle banner with a warning icon:

> I set the limit as 5 just to show the warning

<img width="901" height="298" alt="grafik"
src="https://github.com/user-attachments/assets/a9be8563-1b90-4581-ad15-4e7035b4166e"
/>

## Why

Queries that match thousands of items via wildcard filters (e.g.,
`/.*/`) can return massive amounts of data and potentially overload the
Zabbix server. This proactive warning helps users make informed
decisions about their query scope without adding friction to the normal
query flow.

src/datasource/constants.ts

@@ -49,3 +49,5 @@ export const MIN_SLA_INTERVAL = 3600;
 export const RANGE_VARIABLE_VALUE = 'range_series';
 
 export const DEFAULT_ZABBIX_PROBLEMS_LIMIT = 1001;
+
+export const ITEM_COUNT_WARNING_THRESHOLD = 500;