Allow configuring the Guardian prompt template (#45516)

## What changed

Add `auto_review.experimental_policy_template` to override the Guardian prompt template in `config.toml`. Trim the configured value and ignore it when empty. Prefer the override over the model catalog template, retaining the bundled template as the final fallback.

The template's `{{ tenant_policy_config }}` placeholder is replaced with the resolved Guardian policy.

## Testing

Extend tests to cover template deserialization, trimming, precedence over the catalog template, and rendered policy text in Guardian inference requests.

GitOrigin-RevId: 85b4a8fc193a42735354894203ccbd1f738a3b58

codex-rs/core/config.schema.json

@@ -383,6 +383,10 @@
     },
     "AutoReviewToml": {
       "properties": {
+        "experimental_policy_template": {
+          "description": "Experimental full Guardian prompt template containing the tenant policy placeholder.",
+          "type": "string"
+        },
         "policy": {
           "description": "Additional policy instructions inserted into the guardian prompt.",
           "type": "string"
@@ -7208,4 +7212,4 @@
   },
   "title": "ConfigToml",
   "type": "object"
-}
+}