fix(tools_integrations): Path 4 idempotency keys on structured code only

Drop the generic ``"not found" in error_msg.lower()`` substring fallback
from the Path 4 (config subentry) idempotent-success branch. The check
now relies solely on the structured ``error_code == "not_found"`` from
the HA ``config_entries/subentries/delete`` response.

Generic substring matching was too permissive: HA error messages like
"Repository not found" or "Integration not found" returned by unrelated
failure modes would have silently classified as idempotent success,
masking real errors. The structured ``code: "not_found"`` is the
contract HA uses for this operation; if a future HA version emits a
different code, the tool surfaces SERVICE_CALL_FAILED rather than
mis-classifying — the safer default for a destructive operation.

Adopts Gemini Code Assist finding on PR #1424 (inline thread on the
substring check). Existing Path 4 unit test
(test_delete_helpers_integrations_subentry_not_found_is_idempotent)
mocks the response with ``code: "not_found"`` so it remains the canonical
contract pin; no test changes needed. Sibling-sweep across the other
three idempotent branches confirmed they already key on structured
discriminators (Path 1: state+registry probes, Path 2: lookup-helper
reason field, Path 3: HomeAssistantAPIError.status_code).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Patch76

src/ha_mcp/tools/tools_integrations.py

@@ -1639,13 +1639,18 @@ async def _delete_config_subentry(
         if not isinstance(result, dict) or not result.get("success"):
             error = result.get("error", "Operation failed")
             error_msg = websocket_error_message(error)
-            # Detect "subentry already absent" by HA's not_found code or a
-            # readable message containing "not found" — return idempotent
-            # success in line with the tool's idempotentHint. The HA
-            # config_entries/subentries/delete handler raises with
-            # code="not_found" when entry_id or subentry_id is missing.
+            # Detect "subentry already absent" by HA's structured
+            # ``code="not_found"`` only. A generic ``"not found" in
+            # error_msg`` substring match was rejected because it can
+            # collide with unrelated HA error messages (e.g.
+            # "repository not found", "integration not found") and
+            # silently classify a real failure as idempotent success.
+            # The HA ``config_entries/subentries/delete`` handler raises
+            # with ``code="not_found"`` when entry_id or subentry_id is
+            # missing; if a future HA version uses a different code, we
+            # raise SERVICE_CALL_FAILED instead — safer than mis-classifying.
             error_code = error.get("code") if isinstance(error, dict) else None
-            if error_code == "not_found" or "not found" in error_msg.lower():
+            if error_code == "not_found":
                 return {
                     "success": True,
                     "action": "delete",