Initialize Guardian evidence cursors from thread history state (#60670)

## What changed

Expose history initialization and the effective history mode to thread-ready contributors. Add `EvidenceCursor::at_thread_ready` with paginated-history validation:

- Start new or cleared threads at position zero without requiring an existing rollout.
- Start resumed, forked, or supplied-history threads at the durable tail to avoid backfilling existing evidence.
- Skip the initial `SessionMeta` header when capturing evidence from position zero.

## Testing

Extend evidence tests to cover initialization before a rollout exists, capture of the first original record, rejection of resume without a rollout, and exclusion of existing records on resume.

GitOrigin-RevId: 32aa8e2afb71eb4e882b5f1fb107ac8616b33c8e

codex-rs/core/src/codex_thread.rs

@@ -433,12 +433,15 @@ impl CodexThread {
         if contributors.is_empty() {
             return;
         }
-        let config = self.config().await;
+        let (config, history_initialization, history_mode) =
+            self.session.thread_ready_config().await;
         for contributor in contributors {
             contributor
                 .on_thread_ready(codex_extension_api::ThreadReadyInput {
                     config: config.as_ref(),
                     session_source: &self.session_source,
+                    history_initialization,
+                    history_mode,
                     session_store: &self.session.services.session_extension_data,
                     thread_store: &self.session.services.thread_extension_data,
                 })