Report a published node that has no code of its own

The store answers the new-node template when nothing was ever written for a
node, so such a node ran — returning {} on every call, reporting active and
ok, and saying nothing anywhere. Unreachable through `fluksio sync`, which
writes every body before it publishes; the editor end was open.

A run of a flow holding one is now refused, and the flow carries a
missing_source issue so it is visible before anybody runs it. A draft is
exempt: a node being written legitimately has no published body yet.

The generated client is regenerated for the new issue code, which also
catches up the drift left by earlier backend work (resources, code_digest,
idempotency_key).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/flow/pipeline.py

@@ -58,6 +58,7 @@ class ValidationIssue(BaseModel):
         "node_error",
         "unauthenticated_hook",
         "self_loop_needs_initial",
+        "missing_source",
     ]
     message: str
     flow: str = ""