Migrate from DatasourceAPI to DatasourceWithBackend (#2123)

This PR migrates the use of `DatasourceApi` to `DatasourceWithBackend`,
with this a couple additional improvements were made:

1. Migrate to use `interpolateVariablesInQuery` everywhere instead of
the custom `replaceTemplateVariables` we were using
2. Moves util functions out of `datasource.ts` and into the existing
`utils.ts`

<img width="1261" height="406" alt="Screenshot 2025-11-20 at 11 37
56 AM"
src="https://github.com/user-attachments/assets/9e396cf2-eab0-49d1-958c-963a2e896eba"
/>

Now we can see the `query` calls being made to the backend:
<img width="367" height="102" alt="Screenshot 2025-11-20 at 11 38 18 AM"
src="https://github.com/user-attachments/assets/a5a9a337-7f19-4f7c-9d04-9d30c0216fb2"
/>

Tested:
- By running queries from Explore and Dashboards (with and without
variables)
- By interacting with all the different Editors to make sure `ComboBox`
was working as expected


Next:
Once this is merged, we will next be able to slowly move away from using
the `ZabbixConnector` to make backend datasource calls.

Fixes:
[#131](https://github.com/orgs/grafana/projects/457/views/40?pane=issue&itemId=139450234&issue=grafana%7Coss-big-tent-squad%7C131)

src/datasource/specs/utils.spec.ts

@@ -1,5 +1,6 @@
 import _ from 'lodash';
 import * as utils from '../utils';
+import { replaceTemplateVars, zabbixTemplateFormat } from '../utils';
 
 describe('Utils', () => {
   describe('expandItemName()', () => {
@@ -174,4 +175,90 @@ describe('Utils', () => {
       }
     });
   });
+
+  describe('replaceTemplateVars()', () => {
+    function testReplacingVariable(target, varValue, expectedResult, done) {
+      const templateSrv = {
+        replace: jest.fn((target) => zabbixTemplateFormat(varValue)),
+        getVariables: jest.fn(),
+        containsTemplate: jest.fn(),
+        updateTimeRange: jest.fn(),
+      };
+
+      let result = replaceTemplateVars(templateSrv, target, {});
+      expect(result).toBe(expectedResult);
+      done();
+    }
+
+    /*
+     * Alphanumerics, spaces, dots, dashes and underscores
+     * are allowed in Zabbix host name.
+     * 'AaBbCc0123 .-_'
+     */
+    it('should return properly escaped regex', (done) => {
+      let target = '$host';
+      let template_var_value = 'AaBbCc0123 .-_';
+      let expected_result = '/^AaBbCc0123 \\.-_$/';
+
+      testReplacingVariable(target, template_var_value, expected_result, done);
+    });
+
+    /*
+     * Single-value variable
+     * $host = backend01
+     * $host => /^backend01|backend01$/
+     */
+    it('should return proper regex for single value', (done) => {
+      let target = '$host';
+      let template_var_value = 'backend01';
+      let expected_result = '/^backend01$/';
+
+      testReplacingVariable(target, template_var_value, expected_result, done);
+    });
+
+    /*
+     * Multi-value variable
+     * $host = [backend01, backend02]
+     * $host => /^(backend01|backend01)$/
+     */
+    it('should return proper regex for multi-value', (done) => {
+      let target = '$host';
+      let template_var_value = ['backend01', 'backend02'];
+      let expected_result = '/^(backend01|backend02)$/';
+
+      testReplacingVariable(target, template_var_value, expected_result, done);
+    });
+  });
+
+  describe('replaceVariablesInFuncParams()', () => {
+    it('should interpolate numeric and string params with templateSrv', () => {
+      const replaceMock = jest
+        .fn()
+        .mockImplementation((value) => (value === '42' ? '100' : value.replace('$var', 'result')));
+      const templateSrv = { replace: replaceMock };
+      const scopedVars = { some: 'var' } as any;
+      const functions = [
+        {
+          def: { name: 'test' },
+          params: [42, '$var'],
+        },
+      ];
+
+      const [fn] = utils.replaceVariablesInFuncParams(templateSrv as any, functions as any, scopedVars);
+
+      expect(replaceMock).toHaveBeenCalledWith('42', scopedVars);
+      expect(replaceMock).toHaveBeenCalledWith('$var', scopedVars);
+      expect(fn.params).toEqual([100, 'result']);
+    });
+
+    it('should keep params undefined when function has none', () => {
+      const templateSrv = { replace: jest.fn() };
+      const functions = [{ def: { name: 'noop' } }];
+
+      const [fn] = utils.replaceVariablesInFuncParams(templateSrv as any, functions as any, {} as any);
+
+      expect(fn.params).toBeUndefined();
+      expect(templateSrv.replace).not.toHaveBeenCalled();
+    });
+  });
 });