fix: enable_session_persistence in AgentConfig should be optional (#1012)

# What does this PR do?
This issue was discovered in
https://github.com/meta-llama/llama-stack/pull/1009#discussion_r1947036518.

## Test Plan

This field is no longer required after the change.

[//]: # (## Documentation)
[//]: # (- [ ] Added a Changelog entry if the change is significant)

---------

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>
Co-authored-by: Ashwin Bharambe <ashwin.bharambe@gmail.com>

docs/_static/llama-stack-spec.html

@@ -2724,7 +2724,8 @@
                         "type": "string"
                     },
                     "enable_session_persistence": {
-                        "type": "boolean"
+                        "type": "boolean",
+                        "default": false
                     },
                     "response_format": {
                         "$ref": "#/components/schemas/ResponseFormat"
@@ -2733,8 +2734,7 @@
                 "additionalProperties": false,
                 "required": [
                     "model",
-                    "instructions",
-                    "enable_session_persistence"
+                    "instructions"
                 ]
             },
             "AgentTool": {