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/src/executor/mod.rs

@@ -10,11 +10,11 @@ pub(crate) use runner::ExecutorConfig;
 pub(crate) use runner::normalize_exec_result;
 
 pub(crate) mod linkers {
-    use crate::codex::ExecCommandContext;
     use crate::exec::ExecParams;
     use crate::exec::StdoutStream;
     use crate::executor::backends::ExecutionMode;
     use crate::executor::runner::ExecutionRequest;
+    use crate::tools::context::ExecCommandContext;
 
     pub struct PreparedExec {
         pub(crate) context: ExecCommandContext,

codex-rs/core/src/executor/runner.rs

@@ -6,7 +6,6 @@ use std::time::Duration;
 use super::backends::ExecutionMode;
 use super::backends::backend_for_mode;
 use super::cache::ApprovalCache;
-use crate::codex::ExecCommandContext;
 use crate::codex::Session;
 use crate::error::CodexErr;
 use crate::error::SandboxErr;
@@ -24,6 +23,7 @@ use crate::protocol::AskForApproval;
 use crate::protocol::ReviewDecision;
 use crate::protocol::SandboxPolicy;
 use crate::shell;
+use crate::tools::context::ExecCommandContext;
 use codex_otel::otel_event_manager::ToolDecisionSource;
 
 #[derive(Clone, Debug)]
@@ -303,6 +303,7 @@ pub(crate) fn normalize_exec_result(
             let message = match err {
                 ExecError::Function(FunctionCallError::RespondToModel(msg)) => msg.clone(),
                 ExecError::Codex(e) => get_error_message_ui(e),
+                err => err.to_string(),
             };
             let synthetic = ExecToolCallOutput {
                 exit_code: -1,