Add spans to build_tool_router (#27094)

## Why
- Local profiling shows `append_tool_search_executor` averages ~113ms
per call. Adding a span lets us track this cost as we optimize in
follow-up PRs, either by reducing the work or avoiding repeated rebuilds
when inputs have not changed.
- While we're here, we can add spans to `build_tool_router` and other
sub-calls which code analysis shows may have additional opportunities
for improvement.

## What changed
Add function-level trace spans around `build_tool_router`,
`build_tool_specs_and_registry`, `add_tool_sources`,
`append_tool_search_executor`, and
`build_model_visible_specs_and_registry`

## Verification
Trigger Codex rollout and observe new spans are included

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

@@ -86,6 +86,7 @@ use codex_tools::shell_type_for_model_and_features;
 use std::collections::BTreeMap;
 use std::collections::HashSet;
 use std::sync::Arc;
+use tracing::instrument;
 use tracing::warn;
 
 const MULTI_AGENT_V2_NAMESPACE_DESCRIPTION: &str = "Tools for spawning and managing sub-agents.";
@@ -148,6 +149,7 @@ struct CoreToolPlanContext<'a> {
     wait_agent_timeouts: WaitAgentTimeoutOptions,
 }
 
+#[instrument(level = "trace", skip_all)]
 pub(crate) fn build_tool_router(
     turn_context: &TurnContext,
     params: ToolRouterParams<'_>,
@@ -156,6 +158,7 @@ pub(crate) fn build_tool_router(
     ToolRouter::from_parts(registry, model_visible_specs)
 }
 
+#[instrument(level = "trace", skip_all)]
 fn build_tool_specs_and_registry(
     turn_context: &TurnContext,
     params: ToolRouterParams<'_>,
@@ -186,6 +189,7 @@ fn build_tool_specs_and_registry(
     build_model_visible_specs_and_registry(turn_context, planned_tools)
 }
 
+#[instrument(level = "trace", skip_all)]
 fn build_model_visible_specs_and_registry(
     turn_context: &TurnContext,
     planned_tools: PlannedTools,
@@ -557,6 +561,7 @@ fn code_mode_namespace_descriptions(
     namespace_descriptions
 }
 
+#[instrument(level = "trace", skip_all)]
 fn add_tool_sources(context: &CoreToolPlanContext<'_>, planned_tools: &mut PlannedTools) {
     add_shell_tools(context, planned_tools);
     add_mcp_resource_tools(context, planned_tools);
@@ -837,6 +842,7 @@ fn add_extension_tools(context: &CoreToolPlanContext<'_>, planned_tools: &mut Pl
     );
 }
 
+#[instrument(level = "trace", skip_all)]
 fn append_tool_search_executor(
     context: &CoreToolPlanContext<'_>,
     planned_tools: &mut PlannedTools,