fix: #3286 send realtime output for unknown tool calls

[Aphroq]

Summary

Send a model-visible error tool output when realtime receives a tool call whose name is not available on the current agent.

Known function tools, rejected tools, and handoffs already complete model-visible tool calls with RealtimeModelSendToolOutput. This change makes the unknown-tool branch do the same with start_response=True, while preserving the existing local RealtimeError event for application observers.

Test plan

• uv run pytest tests/realtime/test_session.py -k "unknown_tool_handling"
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3286

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass