Stream Codex custom tools with native input events

Custom (freeform) Codex tools are bridged through Chat Completions as
JSON `{"input": "..."}` functions, but the Chat->Responses stream still
re-emitted them via `response.function_call_arguments.*`, leaking the
JSON wrapper and using event types the Codex client does not route for
freeform tools.

Emit `response.custom_tool_call_input.delta`/`.done` (with the unwrapped
input text) for custom tool calls instead, suppressing the intermediate
function-argument deltas since a partial `{"input":` fragment cannot be
safely unwrapped mid-stream. Custom tool call items now use a `ctc_`
item-id prefix (matching the input events' item_id) consistently across
the streaming and non-streaming paths via a shared
response_tool_call_item_id_from_chat_name helper.

Covered by new streaming and non-streaming custom-tool tests.