Align message-board tools with the multi-agent namespace (#47028)

## Why

Message-board tools hardcoded the `collaboration` namespace and used a different namespace description from the multi-agent tools. This ignored custom namespace configuration and caused conflicts under strict tool collision checking.

## What changed

- Use `multi_agent_v2.tool_namespace` and the shared multi-agent namespace description for message-board tools, resolving the namespace at runtime startup.
- Support unnamespaced tool specifications when no namespace is supplied.
- Remove hardcoded `collaboration` prefixes from message-board notifications.

## Testing

Add app-server integration coverage for the default `collaboration` namespace and a configured `agents` namespace with strict collision checking enabled. Verify that both tool families are advertised together and that `get_channels` executes through the installed extension.

GitOrigin-RevId: fef7be51ea2a84bbdb6d3491606db442ed4e6ea3

codex-rs/core/src/tools/mod.rs

@@ -34,6 +34,7 @@ use codex_utils_output_truncation::TruncationPolicy;
 use codex_utils_output_truncation::formatted_truncate_text;
 use codex_utils_output_truncation::truncate_text;
 pub(crate) use executed_tool_calls::ExecutedToolCalls;
+pub(crate) use multi_agent_tool::MULTI_AGENT_V2_NAMESPACE_DESCRIPTION;
 pub use router::ToolRouter;
 
 /// Legacy boundaries such as hook payloads, telemetry tags, and Responses tool

codex-rs/core/src/tools/multi_agent_tool.rs

@@ -16,7 +16,8 @@ use futures::future::BoxFuture;
 use serde_json::Value;
 use std::sync::Arc;
 
-const MULTI_AGENT_V2_NAMESPACE_DESCRIPTION: &str = "Tools for spawning and managing sub-agents.";
+pub(crate) const MULTI_AGENT_V2_NAMESPACE_DESCRIPTION: &str =
+    "Tools for spawning and managing sub-agents.";
 
 pub(super) fn multi_agent_v2_handler(
     handler: impl CoreToolRuntime + 'static,