Remove ToolName display helper (#21465)

## Why

`ToolName::display()` made it too easy to flatten tool identity and
accidentally compare rendered strings. Tool identity should stay
structural until a legacy string boundary actually requires the
flattened spelling.

## What

- Removes `ToolName::display()` and relies on the existing `Display`
impl for messages and errors.
- Adds structural ordering for `ToolName` and uses it for
sorting/deduping deferred tools.
- Carries `ToolName` through tool/sandbox plumbing, flattening only at
legacy boundaries such as hook payloads, telemetry tags, and Responses
tool names.
- Updates MCP normalization tests to assert `ToolName` structure instead
of rendered strings.

## Testing

- `cargo test -p codex-mcp test_normalize_tools`
- `cargo test -p codex-core unavailable_tool`
- `just fix -p codex-protocol`
- `just fix -p codex-mcp`
- `just fix -p codex-core`

codex-rs/core/src/unified_exec/process_manager.rs

@@ -54,6 +54,7 @@ use codex_protocol::config_types::ShellEnvironmentPolicy;
 use codex_protocol::error::CodexErr;
 use codex_protocol::error::SandboxErr;
 use codex_protocol::protocol::ExecCommandSource;
+use codex_tools::ToolName;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_output_truncation::approx_token_count;
 
@@ -1040,7 +1041,7 @@ impl UnifiedExecProcessManager {
             session: context.session.clone(),
             turn: context.turn.clone(),
             call_id: context.call_id.clone(),
-            tool_name: "exec_command".to_string(),
+            tool_name: ToolName::plain("exec_command"),
         };
         orchestrator
             .run(