test: MCP server tests, benchmarks, and nix build fix

- Add MCP server protocol tests (initialize, tools/list, errors)
- Add database benchmarks (batch inserts, search, children)
- Add sample options.json test fixture
- Fix flake.nix vendor hash for nix build

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1770115704,
+        "narHash": "sha256-KHFT9UWOF2yRPlAnSXQJh6uVcgNcWlFqqiAZ7OVlHNc=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "e6eae2ee2110f3d31110d5c222cd395303343b08",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}