codex: expose enable_state_persistence for agentapi state persistence

[mafredri]

Problem

The codex module doesn't expose enable_state_persistence or pass it through to the module "agentapi" block. Without this, the Coder UI won't preserve chat history across workspace restarts even though the agent itself resumes its session via resume <SESSION_ID>.

The --type codex gap was fixed in #781. This ticket covers the remaining state persistence work.

Desired outcome

Expose enable_state_persistence in main.tf (default true) and pass it through to the module "agentapi" block so the Coder UI preserves chat history across restarts, matching the agent's resumed session.

Version bump

patch

Refs #740
Refs coder/internal#1258

[35C4n0r]

@mafredri, when you make a PR for this can you include this patch: https://gist.github.com/35C4n0r/75114df2e90a4306cd9f5c2fe3e29a58
It's a documentation detail I missed in #781

[mafredri]

@35C4n0r if you have the bandwidth and want to take this ticket on and fix that one at the same time, feel free. Otherwise, got it 👍🏻.