feat(app-server): filter threads by parent (#26662)

## Why

Clients that display or coordinate spawned subagents need an
authoritative snapshot of a thread's immediate spawned children when
they connect to app-server or recover after missing live events.
`thread/list` cannot query by parent, so clients must otherwise scan
unrelated threads or reconstruct relationships from rollout history and
transient events.

The direct spawn relationship already exists in persisted
`thread_spawn_edges` state. Review and Guardian threads do not
participate in that lifecycle and are intentionally outside this
filter's scope.

## What changed

This adds an experimental `parentThreadId` filter to `thread/list`.
Parent-filtered requests return direct spawned children from persisted
state while preserving the existing response shape, explicit filters,
sorting, and timestamp-only cursor behavior. The lookup does not read
rollout transcripts or recursively return descendants.

Supersedes #25112 with the narrower `thread/list` filter approach.

## How it works

1. An experimental client passes a valid thread ID as `parentThreadId`.
2. App-server routes the list through the existing thread-store and
state-database boundaries.
3. SQLite selects threads whose IDs have a direct persisted spawn edge
from that parent.
4. Omitted provider and source filters include all values; explicit
filters keep ordinary `thread/list` semantics.
5. Grandchildren, Review threads, and Guardian threads are excluded.

## Verification

State (144 tests), rollout (69 tests), and focused app-server
thread-list (31 tests) suites passed. Scoped Clippy checks and
repository formatting also passed. Coverage includes direct spawned
children, omitted grandchildren, pagination, malformed IDs, mixed source
kinds, explicit filters, and operation without rollout files.

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -614,6 +614,7 @@ client_request_definitions! {
     },
     ThreadList => "thread/list" {
         params: v2::ThreadListParams,
+        inspect_params: true,
         serialization: None,
         response: v2::ThreadListResponse,
     },

codex-rs/app-server-protocol/src/protocol/v2/thread.rs

@@ -1022,7 +1022,7 @@ pub struct ThreadRollbackResponse {
     pub thread: Thread,
 }
 
-#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS, ExperimentalApi)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct ThreadListParams {
@@ -1062,6 +1062,10 @@ pub struct ThreadListParams {
     /// Optional substring filter for the extracted thread title.
     #[ts(optional = nullable)]
     pub search_term: Option<String>,
+    /// Optional direct parent thread filter.
+    #[experimental("thread/list.parentThreadId")]
+    #[ts(optional = nullable)]
+    pub parent_thread_id: Option<String>,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]