chore: refactor tool handling (#4510)

# Tool System Refactor

- Centralizes tool definitions and execution in `core/src/tools/*`:
specs (`spec.rs`), handlers (`handlers/*`), router (`router.rs`),
registry/dispatch (`registry.rs`), and shared context (`context.rs`).
One registry now builds the model-visible tool list and binds handlers.
- Router converts model responses to tool calls; Registry dispatches
with consistent telemetry via `codex-rs/otel` and unified error
handling. Function, Local Shell, MCP, and experimental `unified_exec`
all flow through this path; legacy shell aliases still work.
- Rationale: reduce per‑tool boilerplate, keep spec/handler in sync, and
make adding tools predictable and testable.

Example: `read_file`
- Spec: `core/src/tools/spec.rs` (see `create_read_file_tool`,
registered by `build_specs`).
- Handler: `core/src/tools/handlers/read_file.rs` (absolute `file_path`,
1‑indexed `offset`, `limit`, `L#: ` prefixes, safe truncation).
- E2E test: `core/tests/suite/read_file.rs` validates the tool returns
the requested lines.

## Next steps:
- Decompose `handle_container_exec_with_params` 
- Add parallel tool calls

codex-rs/tui/src/chatwidget.rs

@@ -111,6 +111,7 @@ use codex_git_tooling::GhostCommit;
 use codex_git_tooling::GitToolingError;
 use codex_git_tooling::create_ghost_commit;
 use codex_git_tooling::restore_ghost_commit;
+use codex_protocol::plan_tool::UpdatePlanArgs;
 use strum::IntoEnumIterator;
 
 const MAX_TRACKED_GHOST_COMMITS: usize = 20;
@@ -508,7 +509,7 @@ impl ChatWidget {
         self.request_redraw();
     }
 
-    fn on_plan_update(&mut self, update: codex_core::plan_tool::UpdatePlanArgs) {
+    fn on_plan_update(&mut self, update: UpdatePlanArgs) {
         self.add_to_history(history_cell::new_plan_update(update));
     }