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/thread-manager-sample/src/main.rs

@@ -213,6 +213,7 @@ fn new_config(model: Option<String>, arg0_paths: Arg0DispatchPaths) -> anyhow::R
         base_instructions_provenance: None,
         developer_instructions: None,
         guardian_policy_config: None,
+        guardian_policy_template: None,
         include_permissions_instructions: false,
         include_apps_instructions: false,
         include_collaboration_mode_instructions: false,