Add organization plugin widgets datasource

[fhacloid]

Summary

• Add cycloid_organization_plugin_widgets datasource backed by ListPluginWidgets(org, placement).
• Expose widget id, type, is_default, plus JSON-encoded placement, widget, and relation payloads.
• Add docs/examples showing plugin widget ID lookup feeding cycloid_organization_nav_order, and correct nav-order schema wording from plugin ID to widget ID.
• Add mapper unit coverage and an acceptance test that installs a plugin before reading side-menu widgets.

Validation

• go test ./... -run TestPluginWidgetsToData -count=1
• go test ./... -run 'Test[^A]|TestA[^c]' -count=1
• go test ./provider -run TestAccOrganizationPluginWidgetsDataSource -short -count=1
• go vet ./...
• go run github.com/hashicorp/terraform-plugin-docs/cmd/tfplugindocs@latest generate --examples-dir examples/ --provider-dir . --provider-name cycloid ./..

Notes

• devenv and golangci-lint are not installed in this cloud image, so lint could not be run here.
• go test ./... -short -count=1 hits existing live-acceptance setup leakage and fails on staging DNS/test-config before this datasource is involved.

Linear Issue: TFPRO-57