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/utils/string/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+edition.workspace = true
+name = "codex-utils-string"
+version.workspace = true
+
+[lints]
+workspace = true

codex-rs/utils/string/src/lib.rs

@@ -0,0 +1,38 @@
+// Truncate a &str to a byte budget at a char boundary (prefix)
+#[inline]
+pub fn take_bytes_at_char_boundary(s: &str, maxb: usize) -> &str {
+    if s.len() <= maxb {
+        return s;
+    }
+    let mut last_ok = 0;
+    for (i, ch) in s.char_indices() {
+        let nb = i + ch.len_utf8();
+        if nb > maxb {
+            break;
+        }
+        last_ok = nb;
+    }
+    &s[..last_ok]
+}
+
+// Take a suffix of a &str within a byte budget at a char boundary
+#[inline]
+pub fn take_last_bytes_at_char_boundary(s: &str, maxb: usize) -> &str {
+    if s.len() <= maxb {
+        return s;
+    }
+    let mut start = s.len();
+    let mut used = 0usize;
+    for (i, ch) in s.char_indices().rev() {
+        let nb = ch.len_utf8();
+        if used + nb > maxb {
+            break;
+        }
+        start = i;
+        used += nb;
+        if start == 0 {
+            break;
+        }
+    }
+    &s[start..]
+}