Unified execution

[jif-oai]

Unified PTY-Based Exec Tool

Note: this requires to have this flag in the config: use_experimental_unified_exec_tool=true

• Adds a PTY-backed interactive exec feature (“unified_exec”) with session reuse via
  session_id, bounded output (128 KiB), and timeout clamping (≤ 60 s).
• Protocol: introduces ResponseItem::UnifiedExec { session_id, arguments, timeout_ms }.
• Tools: exposes unified_exec as a function tool (Responses API); excluded from Chat
  Completions payload while still supported in tool lists.
• Path handling: resolves commands via PATH (or explicit paths), with UTF‑8/newline‑aware
  truncation (truncate_middle).
• Tests: cover command parsing, path resolution, session persistence/cleanup, multi‑session
  isolation, timeouts, and truncation behavior.

[bolinfest]

Nice job and thanks for the tests!

The only comment I feel strongly about is dropping the use of which. If the command does not exist, then the process should fail with ENOENT if we need to treat that case in a special way.