Preserve Guardian sender context for cloud deliveries with orbit IDs (#58946)

## Why

Delegation messages containing `<source_orbit_id>` fail sender provenance parsing, leaving Guardian without the sender's instructions.

## What changed

Accept an optional `<source_orbit_id>` between `<source_thread_id>` and `<input>`, rejecting values containing `<`. Continue to derive sender authority from the source thread ID.

## Testing

Update the cross-host sender context scenario and snapshot to include orbit attribution. The scenario checks that sender evidence reaches Guardian after compaction and rollback without exposing it to the receiving agent.

GitOrigin-RevId: 463b18667384d30297245eedcd0f6bcbf683634e

codex-rs/core/src/agent/control/sender_context.rs

@@ -47,10 +47,18 @@ impl LocalAgentRuntime {
                 .trim()
                 .strip_prefix("<source_thread_id>")?
                 .split_once("</source_thread_id>")?;
-            input
-                .trim()
-                .strip_prefix("<input>")?
-                .strip_suffix("</input>")?;
+            let input = input.trim();
+            // Cloud deliveries include Dot attribution, but authority comes from the source thread.
+            let input = if let Some(rest) = input.strip_prefix("<source_orbit_id>") {
+                let (orbit_id, input) = rest.split_once("</source_orbit_id>")?;
+                if orbit_id.contains('<') {
+                    return None;
+                }
+                input.trim()
+            } else {
+                input
+            };
+            input.strip_prefix("<input>")?.strip_suffix("</input>")?;
             ThreadId::from_string(source)
                 .ok()
                 .filter(|source| *source != receiver_thread_id)