fix: add mistral to MemorySearchSchema provider/fallback unions (#14934)

* fix: add mistral to MemorySearchSchema provider/fallback unions

The Mistral embedding provider was added to the runtime code but the
Zod config schema was not updated, causing config validation to reject
`provider: "mistral"` and `fallback: "mistral"` as invalid input.

* Changelog: add unreleased note for Mistral memory schema fix

---------

Co-authored-by: Drake (Moltbot Dev) <drake@clawd.bot>
Co-authored-by: Vincent Koc <vincentkoc@ieee.org>

src/config/zod-schema.agent-runtime.ts

@@ -490,7 +490,13 @@ export const MemorySearchSchema = z
       .strict()
       .optional(),
     provider: z
-      .union([z.literal("openai"), z.literal("local"), z.literal("gemini"), z.literal("voyage")])
+      .union([
+        z.literal("openai"),
+        z.literal("local"),
+        z.literal("gemini"),
+        z.literal("voyage"),
+        z.literal("mistral"),
+      ])
       .optional(),
     remote: z
       .object({
@@ -516,6 +522,7 @@ export const MemorySearchSchema = z
         z.literal("gemini"),
         z.literal("local"),
         z.literal("voyage"),
+        z.literal("mistral"),
         z.literal("none"),
       ])
       .optional(),