Refactor error handling, client, and tests for Ollama integration

src/constants.ts

@@ -0,0 +1,12 @@
+// Default plugin settings
+
+export const DEFAULT_SETTINGS = {
+  ollamaUrl: 'http://localhost:11434',
+  model: 'llama3',
+  vaultSearchLimit: 3,
+  maxMessageHistory: 50,
+  lastIndexTime: 0,
+};
+
+// Model validation regex - lowercase letters, numbers, dashes, underscores only
+export const MODEL_NAME_REGEX = /^[a-z0-9-_]+$/;