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/core/tests/common/lib.rs

@@ -7,6 +7,9 @@ use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
 use codex_core::config::ConfigToml;
 
+#[cfg(target_os = "linux")]
+use assert_cmd::cargo::cargo_bin;
+
 pub mod responses;
 pub mod test_codex;
 pub mod test_codex_exec;
@@ -17,12 +20,25 @@ pub mod test_codex_exec;
 pub fn load_default_config_for_test(codex_home: &TempDir) -> Config {
     Config::load_from_base_config_with_overrides(
         ConfigToml::default(),
-        ConfigOverrides::default(),
+        default_test_overrides(),
         codex_home.path().to_path_buf(),
     )
     .expect("defaults for test should always succeed")
 }
 
+#[cfg(target_os = "linux")]
+fn default_test_overrides() -> ConfigOverrides {
+    ConfigOverrides {
+        codex_linux_sandbox_exe: Some(cargo_bin("codex-linux-sandbox")),
+        ..ConfigOverrides::default()
+    }
+}
+
+#[cfg(not(target_os = "linux"))]
+fn default_test_overrides() -> ConfigOverrides {
+    ConfigOverrides::default()
+}
+
 /// Builds an SSE stream body from a JSON fixture.
 ///
 /// The fixture must contain an array of objects where each object represents a