feat(llm): mask API keys in UI responses and prevent masked-key leakage in fetch/test/save payloads

- Add mask_api_key() and is_masked_or_placeholder() to llm_provider service
- Return masked keys in all provider CRUD endpoints
- Reject masked/placeholder keys in fetch_models and test_provider_config
- Show masked key with Change button in ProviderConfig.svelte edit form
- Exclude masked keys from fetch-models, test, and submit payloads on frontend
- Update semantics-core skill with clarified complexity tier rules
- Switch agent modes from subagent to all

docs/openapi/litellm/chunks_paths/paths_test.json

@@ -0,0 +1,27 @@
+{
+  "/test": {
+    "get": {
+      "tags": [
+        "health"
+      ],
+      "summary": "Test Endpoint",
+      "description": "[DEPRECATED] use `/health/liveliness` instead.\n\nA test endpoint that pings the proxy server to check if it's healthy.\n\nParameters:\n    request (Request): The incoming request.\n\nReturns:\n    dict: A dictionary containing the route of the request URL.",
+      "operationId": "test_endpoint_test_get",
+      "responses": {
+        "200": {
+          "description": "Successful Response",
+          "content": {
+            "application/json": {
+              "schema": {}
+            }
+          }
+        }
+      },
+      "security": [
+        {
+          "APIKeyHeader": []
+        }
+      ]
+    }
+  }
+}