[codex] Treat max as a first-class reasoning effort

[shijie-oai]

Why

The Bedrock GPT-5.6 catalog advertises max, but Codex treated it as an opaque custom effort. That made the reasoning picker render it as lowercase max while known efforts use productized labels.

Making max a known effort aligns catalog data, parsing, and UI presentation without changing the max wire value or persisted representation.

What changed

• Add first-class ReasoningEffort::Max parsing and serialization.
• Use the typed effort in the Bedrock catalog and render it as Max in the TUI.
• Preserve forward-compatible custom-effort coverage with a genuinely unknown future value.

Before

After

[shijie-oai]

Mark Max as official reasoning effort instead of treating it as a string value from the backend.

[fcoury-oai]

My OCD welcomes this PR. Code looks great, very straightforward.

Codex found one nit that's completely optional.

Approved! 👍

[fcoury-oai]

Nitpick found by Codex:

Now that max has a first-class variant, could we also change reasoning_effort_for_request() in core/src/client.rs to map Ultra to ReasoningEffort::Max instead of Custom("max") (and update its test)? Leaving it unchanged conflicts with Custom’s “unknown effort” contract and means internal request objects still classify a known effort as custom, even though the wire value is the same.

Comment by me: apparently this happens in core/src/client.rs lines 170-175:

fn reasoning_effort_for_request(effort: ReasoningEffortConfig) -> ReasoningEffortConfig {
    match effort {
        ReasoningEffortConfig::Ultra => ReasoningEffortConfig::Custom("max".to_string()),
        effort => effort,
    }
}

This suggestion is a non-blocker of course.