Support remote agent message boards in multi-agent sessions (#49267)

## What changed

Add `features.multi_agent_v2.message_board_remote` with a board URL and a bearer token supplied directly or through an environment variable. When configured, use the remote board in preference to local or in-memory storage, including in ephemeral sessions, while preserving the existing board tools and session identity.

Ignore remote board settings in project-local configuration and use the runtime HTTP client and clock for remote requests.

## Testing

Add a mock-server integration test and snapshot covering an ephemeral session posting through the existing tool, with the expected bearer authentication, session identity, timestamp, and returned post.

GitOrigin-RevId: 263413d90a1ee4c0bdba8c1b08ba9f68bdd7a986

codex-rs/features/src/tests.rs

@@ -774,6 +774,7 @@ non_code_mode_only = true
             wait_agent_enabled: Some(false),
             disable_direct_message: Some(true),
             message_board_in_memory: Some(true),
+            message_board_remote: None,
             non_code_mode_only: Some(true),
         }))
     );