core: add UUIDv7 context window IDs (#28953)

## Why

The token-budget context currently identifies a context window by its
thread-local sequence number. A UUIDv7 gives the model a stable opaque
identity that remains fixed for a window and rotates when compaction or
`new_context` starts the next one.

## What changed

- Preserve the existing monotonic value as `window_number` and add a
UUIDv7 `window_id` to `CompactedItem`.
- Generate and rotate the UUID with auto-compaction window state,
persist it alongside the number, and reconstruct it on resume and
rollback.
- Accept legacy compacted rollout records where the numeric `window_id`
represented the window number.
- Use the UUID only in token-budget context; existing request headers
and metadata continue using `thread_id:window_number`.

## Testing

- `just test -p codex-protocol compacted_item::tests`
- `just test -p codex-core token_budget`

codex-rs/protocol/src/protocol.rs

@@ -2971,13 +2971,17 @@ pub enum RolloutItem {
     EventMsg(EventMsg),
 }
 
-#[derive(Serialize, Deserialize, Clone, Debug, JsonSchema, TS)]
+#[derive(Serialize, Clone, Debug, PartialEq, JsonSchema, TS)]
 pub struct CompactedItem {
     pub message: String,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub replacement_history: Option<Vec<ResponseItem>>,
+    /// Monotonic position of this context window within the thread.
     #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub window_id: Option<u64>,
+    pub window_number: Option<u64>,
+    /// UUIDv7 identity of this context window.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub window_id: Option<String>,
 }
 
 impl From<CompactedItem> for ResponseItem {