feat: surface multi-agent thread limit in spawn description (#19360)

## Summary
- Thread `agent_max_threads` into `ToolsConfig` and
`SpawnAgentToolOptions`.
- Render the configured `max_concurrent_threads_per_session` value in
the MultiAgentV2 `spawn_agent` description.
- Cover the description text in `codex-tools` unit tests and
`codex-core` tool spec tests.

## Validation
- `just fmt`
- `cargo test -p codex-tools`
- `cargo test -p codex-core spawn_agent_description`
- `git diff --check`

## Notes
- `cargo test -p codex-core` was also attempted, but unrelated
environment-sensitive tests failed with the active local environment.
Examples: approvals reviewer defaults observed `AutoReview` instead of
`User`, request-permissions event tests did not emit events, and
proxy-env tests saw `http://127.0.0.1:50604` from the active proxy
environment.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/session/review.rs

@@ -51,6 +51,7 @@ pub(super) async fn spawn_review_thread(
     .with_spawn_agent_usage_hint(config.multi_agent_v2.usage_hint_enabled)
     .with_spawn_agent_usage_hint_text(config.multi_agent_v2.usage_hint_text.clone())
     .with_hide_spawn_agent_metadata(config.multi_agent_v2.hide_spawn_agent_metadata)
+    .with_max_concurrent_threads_per_session(config.agent_max_threads)
     .with_agent_type_description(crate::agent::role::spawn_tool_spec::build(
         &config.agent_roles,
     ));

codex-rs/core/src/session/turn_context.rs

@@ -181,6 +181,7 @@ impl TurnContext {
         .with_spawn_agent_usage_hint(config.multi_agent_v2.usage_hint_enabled)
         .with_spawn_agent_usage_hint_text(config.multi_agent_v2.usage_hint_text.clone())
         .with_hide_spawn_agent_metadata(config.multi_agent_v2.hide_spawn_agent_metadata)
+        .with_max_concurrent_threads_per_session(config.agent_max_threads)
         .with_agent_type_description(crate::agent::role::spawn_tool_spec::build(
             &config.agent_roles,
         ));
@@ -442,6 +443,7 @@ impl Session {
         .with_spawn_agent_usage_hint(per_turn_config.multi_agent_v2.usage_hint_enabled)
         .with_spawn_agent_usage_hint_text(per_turn_config.multi_agent_v2.usage_hint_text.clone())
         .with_hide_spawn_agent_metadata(per_turn_config.multi_agent_v2.hide_spawn_agent_metadata)
+        .with_max_concurrent_threads_per_session(per_turn_config.agent_max_threads)
         .with_agent_type_description(crate::agent::role::spawn_tool_spec::build(
             &per_turn_config.agent_roles,
         ));

codex-rs/core/src/tools/spec_tests.rs

@@ -233,6 +233,7 @@ async fn multi_agent_v2_tools_config() -> ToolsConfig {
         sandbox_policy: &SandboxPolicy::DangerFullAccess,
         windows_sandbox_level: WindowsSandboxLevel::Disabled,
     })
+    .with_max_concurrent_threads_per_session(Some(4))
 }
 
 fn multi_agent_v2_spawn_agent_description(tools_config: &ToolsConfig) -> String {
@@ -749,6 +750,7 @@ async fn spawn_agent_description_omits_usage_hint_when_disabled() {
             \s+Spawned\ agents\ inherit\ your\ current\ model\ by\ default\.\ Omit\ `model`\ to\ use\ that\ preferred\ default;\ set\ `model`\ only\ when\ an\ explicit\ override\ is\ needed\.
             \s+It\ will\ be\ able\ to\ send\ you\ and\ other\ running\ agents\ messages,\ and\ its\ final\ answer\ will\ be\ provided\ to\ you\ when\ it\ finishes\.
             \s+The\ new\ agent's\ canonical\ task\ name\ will\ be\ provided\ to\ it\ along\ with\ the\ message\.
+            \s+This\ session\ is\ configured\ with\ `max_concurrent_threads_per_session\ =\ 4`\ for\ concurrently\ open\ agent\ threads\.
             \s*$
         "#,
         &description,
@@ -774,6 +776,7 @@ async fn spawn_agent_description_uses_configured_usage_hint_text() {
             \s+Spawned\ agents\ inherit\ your\ current\ model\ by\ default\.\ Omit\ `model`\ to\ use\ that\ preferred\ default;\ set\ `model`\ only\ when\ an\ explicit\ override\ is\ needed\.
             \s+It\ will\ be\ able\ to\ send\ you\ and\ other\ running\ agents\ messages,\ and\ its\ final\ answer\ will\ be\ provided\ to\ you\ when\ it\ finishes\.
             \s+The\ new\ agent's\ canonical\ task\ name\ will\ be\ provided\ to\ it\ along\ with\ the\ message\.
+            \s+This\ session\ is\ configured\ with\ `max_concurrent_threads_per_session\ =\ 4`\ for\ concurrently\ open\ agent\ threads\.
             \s+Custom\ delegation\ guidance\ only\.
             \s*$
         "#,