[codex] Treat max as a first-class reasoning effort (#30467)

## 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
<img width="559" height="124" alt="Screenshot 2026-06-28 at 12 08 47 PM"
src="https://github.com/user-attachments/assets/7c43cf4f-020b-4605-9239-0a9c97eb7364"
/>

### After
<img width="558" height="107" alt="Screenshot 2026-06-28 at 12 09 10 PM"
src="https://github.com/user-attachments/assets/b9cc5ded-c940-43b4-b024-bba25abe0a17"
/>

codex-rs/core/src/client.rs

@@ -169,7 +169,7 @@ pub(crate) struct CompactConversationRequestSettings {
 
 fn reasoning_effort_for_request(effort: ReasoningEffortConfig) -> ReasoningEffortConfig {
     match effort {
-        ReasoningEffortConfig::Ultra => ReasoningEffortConfig::Custom("max".to_string()),
+        ReasoningEffortConfig::Ultra => ReasoningEffortConfig::Max,
         effort => effort,
     }
 }