[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/model-provider/src/amazon_bedrock/catalog.rs

@@ -70,7 +70,7 @@ fn gpt_5_6_bedrock_model(bedrock_slug: &str, display_name: &str, priority: i32)
     model
         .supported_reasoning_levels
         .push(ReasoningEffortPreset {
-            effort: ReasoningEffort::Custom("max".to_string()),
+            effort: ReasoningEffort::Max,
             description: "Maximum reasoning depth for the hardest problems".to_string(),
         });
     model
@@ -148,7 +148,7 @@ mod tests {
             expected
                 .supported_reasoning_levels
                 .push(ReasoningEffortPreset {
-                    effort: ReasoningEffort::Custom("max".to_string()),
+                    effort: ReasoningEffort::Max,
                     description: "Maximum reasoning depth for the hardest problems".to_string(),
                 });