Move string truncation helpers into codex-utils-string (#15572)

- move the shared byte-based middle truncation logic from `core` into
`codex-utils-string`
- keep token-specific truncation in `codex-core` so rollout can reuse
the shared helper in the next stacked PR

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/utils/string/src/truncate/tests.rs

@@ -0,0 +1,71 @@
+use super::split_string;
+use super::truncate_middle_chars;
+use super::truncate_middle_with_token_budget;
+use pretty_assertions::assert_eq;
+
+#[test]
+fn split_string_works() {
+    assert_eq!(split_string("hello world", 5, 5), (1, "hello", "world"));
+    assert_eq!(split_string("abc", 0, 0), (3, "", ""));
+}
+
+#[test]
+fn split_string_handles_empty_string() {
+    assert_eq!(split_string("", 4, 4), (0, "", ""));
+}
+
+#[test]
+fn split_string_only_keeps_prefix_when_tail_budget_is_zero() {
+    assert_eq!(split_string("abcdef", 3, 0), (3, "abc", ""));
+}
+
+#[test]
+fn split_string_only_keeps_suffix_when_prefix_budget_is_zero() {
+    assert_eq!(split_string("abcdef", 0, 3), (3, "", "def"));
+}
+
+#[test]
+fn split_string_handles_overlapping_budgets_without_removal() {
+    assert_eq!(split_string("abcdef", 4, 4), (0, "abcd", "ef"));
+}
+
+#[test]
+fn split_string_respects_utf8_boundaries() {
+    assert_eq!(split_string("😀abc😀", 5, 5), (1, "😀a", "c😀"));
+
+    assert_eq!(split_string("😀😀😀😀😀", 1, 1), (5, "", ""));
+    assert_eq!(split_string("😀😀😀😀😀", 7, 7), (3, "😀", "😀"));
+    assert_eq!(split_string("😀😀😀😀😀", 8, 8), (1, "😀😀", "😀😀"));
+}
+
+#[test]
+fn truncate_with_token_budget_returns_original_when_under_limit() {
+    let s = "short output";
+    let limit = 100;
+    let (out, original) = truncate_middle_with_token_budget(s, limit);
+    assert_eq!(out, s);
+    assert_eq!(original, None);
+}
+
+#[test]
+fn truncate_with_token_budget_reports_truncation_at_zero_limit() {
+    let s = "abcdef";
+    let (out, original) = truncate_middle_with_token_budget(s, 0);
+    assert_eq!(out, "…2 tokens truncated…");
+    assert_eq!(original, Some(2));
+}
+
+#[test]
+fn truncate_middle_tokens_handles_utf8_content() {
+    let s = "😀😀😀😀😀😀😀😀😀😀\nsecond line with text\n";
+    let (out, tokens) = truncate_middle_with_token_budget(s, 8);
+    assert_eq!(out, "😀😀😀😀…8 tokens truncated… line with text\n");
+    assert_eq!(tokens, Some(16));
+}
+
+#[test]
+fn truncate_middle_bytes_handles_utf8_content() {
+    let s = "😀😀😀😀😀😀😀😀😀😀\nsecond line with text\n";
+    let out = truncate_middle_chars(s, 20);
+    assert_eq!(out, "😀😀…21 chars truncated…with text\n");
+}