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/tool_config.rs

@@ -107,6 +107,7 @@ pub struct ToolsConfig {
     pub hide_spawn_agent_metadata: bool,
     pub spawn_agent_usage_hint: bool,
     pub spawn_agent_usage_hint_text: Option<String>,
+    pub max_concurrent_threads_per_session: Option<usize>,
     pub default_mode_request_user_input: bool,
     pub experimental_supported_tools: Vec<String>,
     pub agent_jobs_tools: bool,
@@ -228,6 +229,7 @@ impl ToolsConfig {
             hide_spawn_agent_metadata: false,
             spawn_agent_usage_hint: true,
             spawn_agent_usage_hint_text: None,
+            max_concurrent_threads_per_session: None,
             default_mode_request_user_input: include_default_mode_request_user_input,
             experimental_supported_tools: model_info.experimental_supported_tools.clone(),
             agent_jobs_tools: include_agent_jobs,
@@ -259,6 +261,14 @@ impl ToolsConfig {
         self
     }
 
+    pub fn with_max_concurrent_threads_per_session(
+        mut self,
+        max_concurrent_threads_per_session: Option<usize>,
+    ) -> Self {
+        self.max_concurrent_threads_per_session = max_concurrent_threads_per_session;
+        self
+    }
+
     pub fn with_allow_login_shell(mut self, allow_login_shell: bool) -> Self {
         self.allow_login_shell = allow_login_shell;
         self