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/Cargo.toml

@@ -65,6 +65,7 @@ members = [
     "utils/sleep-inhibitor",
     "utils/approval-presets",
     "utils/oss",
+    "utils/output-truncation",
     "utils/fuzzy-match",
     "utils/stream-parser",
     "codex-client",
@@ -154,6 +155,7 @@ codex-utils-home-dir = { path = "utils/home-dir" }
 codex-utils-image = { path = "utils/image" }
 codex-utils-json-to-toml = { path = "utils/json-to-toml" }
 codex-utils-oss = { path = "utils/oss" }
+codex-utils-output-truncation = { path = "utils/output-truncation" }
 codex-utils-pty = { path = "utils/pty" }
 codex-utils-readiness = { path = "utils/readiness" }
 codex-utils-rustls-provider = { path = "utils/rustls-provider" }