fix: extract thread_id from Codex events for Session.send multi-turn

[joshrotenberg]

Summary

• Fix Session.send/3 silently dropping conversation history on every follow-up turn against codex-cli ≥ 0.119. extract_session_id/1 was looking up "session_id" in events, but Codex emits its thread identifier as "thread_id" in the first thread.started event. The captured id was always nil, new_session_id fell through to the stored (nil) session.session_id, and the next call to Session.send/3 dispatched a fresh Exec (not ExecResume) with no memory of prior turns.
• Change the lookup to prefer "thread_id", fall back to "session_id" for defensive compatibility with older Codex versions or forks.
• Expose extract_session_id/1 with @doc false so the capture logic can be covered by unit tests (same pattern as Command.shell_cmd_args/3).

Closes #40.

Test plan

• mix test — 222 tests pass (5 new regression tests for extract_session_id/1)
• mix dialyzer — clean
• Live multi-turn verification against codex-cli 0.119.0:

  --- Turn 1 ---
  session_id after turn 1: "019d79cf-c221-7702-924a-b10fd5a77b44"
  result1 stdout first line: {"type":"thread.started","thread_id":"019d79cf-c221-7702-924a-b10fd5a77b44"}
  
  --- Turn 2 ---
  result2 stdout: {"type":"thread.started","thread_id":"019d79cf-c221-7702-924a-b10fd5a77b44"}
  {"type":"turn.started"}
  {"type":"item.completed","item":{"id":"item_0","type":"agent_message","text":"42"}}
  {"type":"turn.completed","usage":{"input_tokens":27980,"cached_input_tokens":18304,"output_tokens":33}}
  
  SUCCESS: turn 2 remembered the number 42 -> multi-turn via Session.send works
  

  Before this fix: session_id was nil after turn 1, and turn 2 ran as a fresh thread with no knowledge of the number.
  After this fix: session_id is a real thread id after turn 1, turn 2 emits the same thread id in its thread.started event (confirming Codex resumed the existing thread), and the assistant correctly answered "42".

Regression tests

New describe "extract_session_id/1" block in test/codex_wrapper/session_test.exs covers:

• thread_id extracted from thread.started
• session_id fallback when thread_id is absent
• thread_id preferred when both are present
• nil when neither is present
• nil on an empty event list

Background

Discovered while filing #40 as a follow-up to the chore/add-dialyxir work — see the Session.stream/3 fix in #36. The two functions had the same underlying field-name bug, but only Session.stream/3 was visible to dialyzer because of the make_ref() type abuse. Session.send/3 compiled cleanly but failed at runtime without any indication.