Rename setup_experience_platforms to singular comma-separated setup_experience_platform (#49245)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
remove if NA -->
**Related issue:** #43667

# Summary

Renames the unreleased GitOps field `setup_experience_platforms` to
singular `setup_experience_platform`, accepting a comma-separated string
of `darwin`/`linux` (rejecting the `macos` alias) to match the
query/policy/label `platform` convention.

# Checklist for submitter

- [ ] Input data is properly validated, `SELECT *` is avoided, SQL
injection is prevented (using placeholders for values in statements), JS
inline code is prevented especially for url redirects, and untrusted
data interpolated into shell scripts/commands is validated against shell
metacharacters.

## Testing

- [ ] Added/updated automated tests
- [ ] QA'd all new/changed functionality manually

## New Fleet configuration settings

- [ ] Verified that the setting is exported via `fleetctl
generate-gitops`
- [ ] Verified the setting is documented in a separate PR to [the GitOps
documentation](https://github.com/fleetdm/fleet/blob/main/docs/Configuration/yaml-files.md#L485)
- [ ] Verified that the setting is cleared on the server if it is not
supplied in a YAML file (or that it is documented as being optional)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Improvements**
* Updated software setup-experience platform configuration to use a
single comma-separated `setup_experience_platform` value.
* Platform values are normalized for casing and whitespace,
deduplicated, and validated against supported platforms.
* macOS setup selections now use the canonical `darwin` value; the
`macos` alias is rejected.
* GitOps-generated configurations now use the updated field name and
platform format.
* **Bug Fixes**
* Improved validation messages for invalid setup-experience platform
values.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

cmd/fleetctl/fleetctl/generate_gitops.go

@@ -2002,8 +2002,8 @@ func (cmd *GenerateGitopsCommand) generateSoftware(filePath string, teamID uint,
 
 	setupSoftwareBySoftwareTitle := make(map[uint]struct{})
 	setupSoftwareByPlatformAndAppID := make(map[string]struct{})
-	// Emitted as setup_experience_platforms so a UI-set non-native selection
-	// round-trips through generate → apply unchanged.
+	// Emitted as setup_experience_platform (comma-separated) so a UI-set
+	// non-native selection round-trips through generate → apply unchanged.
 	crossPlatformSelectionsByTitleID := make(map[uint][]string)
 
 	// Fill in InstallDuringSetup for software, as that information is only available
@@ -2043,7 +2043,9 @@ func (cmd *GenerateGitopsCommand) generateSoftware(filePath string, teamID uint,
 			if pkg.Platform == fleet.CanonicalPlatform(crossTarget) {
 				continue
 			}
-			crossPlatformSelectionsByTitleID[t.ID] = append(crossPlatformSelectionsByTitleID[t.ID], crossTarget)
+			// Emit the canonical platform token ("darwin", not "macos") to match
+			// the query/policy/label `platform` convention.
+			crossPlatformSelectionsByTitleID[t.ID] = append(crossPlatformSelectionsByTitleID[t.ID], fleet.CanonicalPlatform(crossTarget))
 		}
 	}
 
@@ -2353,7 +2355,7 @@ func (cmd *GenerateGitopsCommand) generateSoftware(filePath string, teamID uint,
 				softwareSpec["setup_experience"] = true
 			}
 			if crosses, ok := crossPlatformSelectionsByTitleID[softwareTitle.ID]; ok && len(crosses) > 0 {
-				softwareSpec["setup_experience_platforms"] = crosses
+				softwareSpec["setup_experience_platform"] = strings.Join(crosses, ",")
 			}
 		} else {
 			platformAndAppID := softwareTitle.AppStoreApp.VPPAppID.String()