fix: align channel data structure with backend API specification

[VikaCep]

Description

Aligns the frontend's k6 channel data structure with the new backend API specification. The channel information is now stored at the root level (channels.k6) instead of nested within check type settings. See https://github.com/grafana/synthetic-monitoring/issues/408#issuecomment-3595507029 for its definition and discussion.

Backend Context

The backend API now expects/returns channel data in this format:

Request (create/update check):

{
  "channels": {
    "k6": {
      "id": "v1"
    }
  }
}

Response (fetching checks):

{
  "channels": {
    "k6": {
      "id": "v1",
      "name": "v1.x",
      "default": true,
      "manifest": "^1.0.0",
      "deprecatedAfter": "2027-06-01T00:00:00Z"
    }
  }
}

Related backend PR: https://github.com/grafana/synthetic-monitoring-api/pull/1876

[github-actions]

Script size changes

Name	+/-	Main	This PR	Outcome
[263.js]	+4.24%	1,646.45 kB	1,716.20 kB	✅
[234.js]	New file	-	804.37 kB	✅
[datasource/module.js]	-0.16%	24.56 kB	24.52 kB	✅
[692.js]	-3.66%	20.57 kB	19.82 kB	✅
[663.js]	=	5.84 kB	5.84 kB	✅
[module.js]	+0.08%	4.85 kB	4.86 kB	✅
[88.js]	Deleted file	984.46 kB	-

Totals

Name	+/-	Main	This PR	Outcome
[Scripts]	-4.14%	2,686.74 kB	2,575.61 kB	✅
[Non-script Assets]	+0.02%	2,671.09 kB	2,671.65 kB	✅
[All]	-2.06%	5,357.82 kB	5,247.26 kB	✅

Generated by 🚫 dangerJS against 2ca38a4

[ckbedwell]

LGTM! I much prefer this to the previous schema.