bug: spawn_agent returns "unsupported call" for custom providers (flat dotted tool names)

Summary

With certain (non-OpenAI / custom) model providers, namespaced tool calls arrive on the wire flattened into a single dotted name with namespace: None, e.g. multi_agent_v1.spawn_agent.

The router builds the lookup key from that name under the default namespace, so the registered (namespace, name) handler is missed and the model receives unsupported call: multi_agent_v1.spawn_agent.

Repro

# config.toml
      "experimental_supported_tools": [
        "multi_agent_v1",
        "multi_agent_v2"
      ]

• Provider: Ollama (huihui_ai/Qwen3.8-abliterated:27b, provider ollama-launch-codex-app)
• Before: model asks to spawn_agent → unsupported call: multi_agent_v1.spawn_agent
• After fix: sub-agent is created and returns normally

Root cause

In ToolRouter::build_tool_call (FunctionCall branch, codex-rs/core/src/tools/router.rs), when no explicit namespace is present, the name is used as-is under the default namespace. For dotted names like multi_agent_v1.spawn_agent, this misses the handler registered under (namespace: "multi_agent_v1", name: "spawn_agent").

Fix

Split a dotted name at the first dot into (namespace, name) only when no explicit namespace is present:

• explicit namespace always wins (unchanged behavior)
• plain non-dotted names keep the default namespace (unchanged)
• only dotted names with an empty/absent namespace are split

Diff: codex-rs/core/src/tools/router.rs +15/−1

Test: Added build_tool_call_splits_flat_dotted_name_when_namespace_is_absent (codex-rs/core/src/tools/router_tests.rs, +65) covering:

1. flat dotted + no namespace → (multi_agent_v1, spawn_agent)
2. plain name + no namespace → default namespace (functions)
3. explicit namespace + spawn_agent → explicit namespace preserved

cargo test -p codex-core --lib build_tool_call_splits_flat_dotted_name_when_namespace_is_absent
# ... ok

Verification

Verified end-to-end in the desktop app: sub-agent created and waited on successfully.

Notes

• Fixes a whole class of namespaced tools (not just multi_agent_v1), e.g. any multi_agent_v1.* / multi_agent_v2.* / node_repl.js flat names from providers that flatten the namespace into the name.
• Does not touch the CustomToolCall path or explicit-namespace routing.

PR

I've prepared a PR in my fork: fedorbrain/codex#1

Since openai/codex limits PRs to collaborators, I can either:

1. Open a cross-fork PR if added as a collaborator, or
2. You can cherry-pick the commit a0e8240 from my fork.

Start in codex-rs/core/src/tools/router.rs at ToolRouter::build_tool_call and review the related test in codex-rs/core/src/tools/router_tests.rs. Run the named cargo test to verify flat dotted names split only when the namespace is absent, while plain names and explicit namespaces retain their existing routing behavior.