feat(gatherer): Update json schema response for gatherer (#2595)

Signed-off-by: Javier Rodriguez <javier@chainloop.dev>

Author: javirln

internal/schemavalidators/external_schemas/runnercontext/runner-context-response-0.1.schema.json

@@ -8,6 +8,10 @@
                     "type": "string",
                     "description": "Unique identifier for the runner context schema version"
                 },
+                "generated_at": {
+                    "type": "string",
+                    "description": "Timestamp when the runner context was generated (RFC3339 format)"
+                },
                 "errors": {
                     "items": {
                         "properties": {