fix(workflow): answer when the product does not exist

MCP does not report a tool failure as a JSON-RPC error. It reports it as a
successful result carrying isError: true, with the reason as text. The node
only checked `error`, so it read the failure as data and went on to JSON.parse
the sentence "HTTP call failed with status 404" as a product. The throw is what
made the webhook answer nothing at all.

Now checks result.isError, distinguishes a missing product from any other tool
failure, and guards every parse of a payload it did not construct. Through the
facade, escalate: true becomes a task in state failed rather than a completed
one.

Also corrects this file: the earlier note said the run completed in success
with every node executing. It did not — that was the wrong execution. n8n's
executions list did not return them newest-first as assumed, and the conclusion
drawn from it sent the search to the wrong place.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

workflows/README.md

@@ -51,13 +51,40 @@ The regex is loose after the underscore on purpose — a reference that looks
 valid but does not exist must reach the catalog and come back as "no such
 product", because answering "you named no product" would be false.
 
-### Known gap
+### The one that took longest
 
-A product code that does not exist returns an **empty body** rather than that
-message. The run completes, every node executes including the one that composes
-the answer, and the response is still empty — so the fault is between the
-composing node and the webhook response, not in the decision logic.
-`alwaysOutputData` on the read node did not change it.
+A product code that does not exist returned an **empty body**, and the reason is
+worth keeping.
 
-For an autonomous agent, silence is the worst possible answer. Not shipped as
-acceptable — recorded as unfinished.
+MCP does not report a tool failure as a JSON-RPC `error`. It reports it as a
+perfectly successful **result** carrying `isError: true`, with the reason as
+text:
+
+```
+data: {"jsonrpc":"2.0","result":{"content":[{"type":"text",
+       "text":"HTTP call failed with status 404: {\"detail\":\"Not found.\"}"}],
+       "isError":true},"id":2}
+```
+
+A client that only checks `rpc.error` therefore reads a failure as data. This
+one went on to `JSON.parse` the sentence "HTTP call failed with status 404" as
+if it were a product, threw, and the webhook answered nothing at all.
+
+Two lessons, both now in the node:
+
+- **check `result.isError`, not just `error`** — they mean different things: a
+  protocol failure versus a tool that ran and failed;
+- **guard every `JSON.parse` of a payload you did not construct.** The throw is
+  what turned a wrong answer into no answer, which for an autonomous agent is
+  the worse of the two.
+
+Now:
+
+```
+"Peut-on promettre S99_NOPE ?"
+  → {"answer":"There is no product S99_NOPE in the catalog.","escalate":true}
+```
+
+and through the A2A facade, `escalate: true` becomes a task in state **failed**
+rather than a completed one — the honest shape for "a human has to look at
+this".