Make multi-agent v2 tool namespace configurable (#23147)

## Summary
- Add `features.multi_agent_v2.tool_namespace` with config/schema
validation for Responses-compatible namespace values.
- Thread the resolved namespace into `ToolsConfig` for normal turns and
review turns.
- Wrap MultiAgentV2 tool specs and registry names in the configured
namespace when namespace tools are supported, while falling back to the
plain tool names when they are not.

## Validation
- `just fmt`
- `just write-config-schema`
- `cargo test -p codex-features multi_agent_v2_feature_config --
--nocapture`
- `cargo test -p codex-core test_build_specs_multi_agent_v2 --
--nocapture`
- `cargo test -p codex-core multi_agent_v2_config -- --nocapture`
- `cargo test -p codex-core
multi_agent_v2_rejects_invalid_tool_namespace -- --nocapture`
- `cargo test -p codex-tools`
- `git diff --check`

codex-rs/core/config.schema.json

@@ -1516,6 +1516,12 @@
         "subagent_usage_hint_text": {
           "type": "string"
         },
+        "tool_namespace": {
+          "maxLength": 64,
+          "minLength": 1,
+          "pattern": "^[a-zA-Z0-9_-]+$",
+          "type": "string"
+        },
         "usage_hint_enabled": {
           "type": "boolean"
         },