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/tools/src/agent_tool_tests.rs

@@ -40,6 +40,7 @@ fn spawn_agent_tool_v2_requires_task_name_and_lists_visible_models() {
         hide_agent_type_model_reasoning: false,
         include_usage_hint: true,
         usage_hint_text: None,
+        max_concurrent_threads_per_session: Some(4),
     });
 
     let ToolSpec::Function(ResponsesApiTool {
@@ -61,6 +62,7 @@ fn spawn_agent_tool_v2_requires_task_name_and_lists_visible_models() {
         .expect("spawn_agent should use object params");
     assert!(description.contains("Spawns an agent to work on the specified task."));
     assert!(description.contains("The spawned agent will have the same tools as you"));
+    assert!(description.contains("`max_concurrent_threads_per_session = 4`"));
     assert!(description.contains(SPAWN_AGENT_INHERITED_MODEL_GUIDANCE));
     assert!(
         description
@@ -101,6 +103,7 @@ fn spawn_agent_tool_v1_keeps_legacy_fork_context_field() {
         hide_agent_type_model_reasoning: false,
         include_usage_hint: true,
         usage_hint_text: None,
+        max_concurrent_threads_per_session: None,
     });
 
     let ToolSpec::Function(ResponsesApiTool { parameters, .. }) = tool else {