Add Guardian internal session support (#39994)

## What changed

- Add an extension API for spawning host-owned internal sessions and a
  `ThreadReadyInput` lifecycle callback that runs after thread registration.
- Add Guardian reviewer session scaffolding that records the parent thread and
  effective model, plus the under-development `guardian_ext` feature flag.
- Start internal sessions with fresh history while preserving parent lineage,
  shared session controls, and internal-thread visibility rules.
- Scope internal-session prompt cache keys to their source and parent thread,
  and expose `guardian` as an internal session source.

## Testing

- Cover internal-session spawning, parent metadata, history isolation, prompt
  cache keys, and extension spawner argument forwarding.

GitOrigin-RevId: 682dae80397d62bb36247796b447042e760ca364

codex-rs/protocol/src/protocol.rs

@@ -2650,6 +2650,7 @@ impl FromStr for ThreadSource {
 #[ts(rename_all = "snake_case")]
 pub enum InternalSessionSource {
     MemoryConsolidation,
+    Guardian,
 }
 
 #[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, JsonSchema, TS)]
@@ -2822,6 +2823,7 @@ impl fmt::Display for InternalSessionSource {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             InternalSessionSource::MemoryConsolidation => f.write_str("memory_consolidation"),
+            InternalSessionSource::Guardian => f.write_str("guardian"),
         }
     }
 }