[tool search] support namespaced deferred dynamic tools (#18413)

Deferred dynamic tools need to round-trip a namespace so a tool returned
by `tool_search` can be called through the same registry key that core
uses for dispatch.

This change adds namespace support for dynamic tool specs/calls,
persists it through app-server thread state, and routes dynamic tool
calls by full `ToolName` while still sending the app the leaf tool name.
Deferred dynamic tools must provide a namespace; non-deferred dynamic
tools may remain top-level.

It also introduces `LoadableToolSpec` as the shared
function-or-namespace Responses shape used by both `tool_search` output
and dynamic tool registration, so dynamic tools use the same wrapping
logic in both paths.

Validation:
- `cargo test -p codex-tools`
- `cargo test -p codex-core tool_search`

---------

Co-authored-by: Sayan Sisodiya <sayan@openai.com>

codex-rs/protocol/src/protocol.rs

@@ -2441,6 +2441,9 @@ pub struct DynamicToolCallResponseEvent {
     pub call_id: String,
     /// Turn ID that this dynamic tool call belongs to.
     pub turn_id: String,
+    /// Dynamic tool namespace, when one was provided.
+    #[serde(default)]
+    pub namespace: Option<String>,
     /// Dynamic tool name.
     pub tool: String,
     /// Dynamic tool call arguments.