feat: add SessionCache trait with memory and redis backends for model affinity

Agent-Logs-Url: https://github.com/katanemo/plano/sessions/6a75240b-4578-409d-b8c7-eff47dba8a03

Co-authored-by: adilhafeez <13196462+adilhafeez@users.noreply.github.com>

config/plano_config_schema.yaml

@@ -441,6 +441,20 @@ properties:
         minimum: 1
         maximum: 10000
         description: Maximum number of session-pinned routing cache entries. Default 10000.
+      session_cache:
+        type: object
+        description: Session cache backend configuration. Defaults to in-memory if omitted.
+        properties:
+          type:
+            type: string
+            enum: [memory, redis]
+            description: "Cache backend type. 'memory' (default) is in-process; 'redis' is shared across replicas."
+          url:
+            type: string
+            description: "Redis connection URL (e.g. redis://localhost:6379). Required when type is 'redis'."
+        required:
+          - type
+        additionalProperties: false
     additionalProperties: false
   state_storage:
     type: object