fix(deploy): self-diagnosing sha sensor + drop invalid not-key [#734]

Gitea response came back a dict (error body) - sensor state now shows
the truncated body so one restart identifies it. not: rejected by
trigger schema; guard lives in the condition template already.
Detail: https://projects.knownelement.com/issues/734

packages/deploy_pipeline.yaml

@@ -15,5 +15,5 @@ rest:
     sensor:
       - name: PFV deploy gitea master head
         unique_id: pfv_deploy_gitea_master_head
-        availability: "{{ value_json is defined and value_json | length > 0 }}"
-        value_template: "{{ value_json[0].sha }}"
+        availability: "{{ value_json is defined and value_json is list and value_json | length > 0 }}"
+        value_template: "{{ value_json[0].sha if value_json is list and value_json | length > 0 else (value_json | string | truncate(90)) }}"