feat: add Mistral embedding provider

Adds support for Mistral embeddings with:
- Mistral embedding client and provider
- Batch processing with concurrency control
- Auto-fallback support
- Tests for remote overrides and key resolution

src/memory/embeddings.test.ts

@@ -1,5 +1,6 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
 
+import { DEFAULT_MISTRAL_EMBEDDING_MODEL } from "./embeddings-mistral.js";
 import { DEFAULT_GEMINI_EMBEDDING_MODEL } from "./embeddings-gemini.js";
 
 vi.mock("../agents/model-auth.js", () => ({