fix(sl): classify semantic-query request rejections as expected, not faults (#339)

The daemon rejects an invalid semantic-query request (unknown source,
ambiguous measure, no join path) with a plain ValueError; the Node compute
port now maps the daemon's exit code 3 / HTTP 400 to KtxExpectedError so these
routine, caller-driven rejections stay out of Error Tracking.

A dedicated SemanticLayerRequestError(ValueError) is raised only for engine
rejections and routed through both daemon transports and the HTTP handler.
Because pydantic v2 ValidationError subclasses ValueError, malformed sources or
responses (contract faults) are kept as faults: they are reported and mapped to
exit 1 / HTTP 500 / plain Error on every path. Non-object stdin is likewise a
fault (exit 1), not exit 3.

python/ktx-daemon/src/ktx_daemon/app.py

@@ -12,6 +12,7 @@ from typing import Any
 
 from fastapi import FastAPI, HTTPException, Request
 from fastapi.responses import JSONResponse, Response
+from pydantic import ValidationError
 
 from ktx_daemon import VERSION
 from ktx_daemon.code_execution import (
@@ -268,6 +269,14 @@ def create_app(
     ) -> SemanticLayerQueryResponse:
         try:
             return query_semantic_layer(request)
+        except ValidationError as error:
+            # A malformed source or response is a ktx contract fault, not a
+            # caller rejection; surface it as a server fault (500) so the Node
+            # client does not classify it as an expected query rejection, matching
+            # the subprocess transport (exit 1) and query_semantic_layer's own
+            # report. ValidationError subclasses ValueError, so catch it first.
+            logger.exception("Semantic query failed on a malformed source")
+            raise HTTPException(status_code=500, detail=str(error)) from error
         except ValueError as error:
             logger.warning("Semantic query rejected: %s", error)
             raise HTTPException(status_code=400, detail=str(error)) from error

python/ktx-daemon/tests/test_app.py

@@ -424,6 +424,24 @@ def test_semantic_query_endpoint_maps_value_error_to_400() -> None:
     assert "missing.order_count" in response.json()["detail"]
 
 
+def test_semantic_query_endpoint_maps_malformed_source_to_fault() -> None:
+    client = TestClient(create_app(), raise_server_exceptions=False)
+    invalid_source = {
+        key: value for key, value in ORDERS_SOURCE.items() if key != "table"
+    }
+
+    response = client.post(
+        "/semantic-layer/query",
+        json={
+            "sources": [invalid_source],
+            "dialect": "postgres",
+            "query": {"measures": ["orders.order_count"], "dimensions": []},
+        },
+    )
+
+    assert response.status_code == 500
+
+
 def test_semantic_validate_endpoint_returns_structured_validation() -> None:
     client = TestClient(create_app())
     invalid_source = {