[VA-496] Adding interruptible configuration to Transfer and EndCall tools

[lucyliulee]

What does this PR do?

• Add configurable interruptible field to EndCallTool, TransferCallTool, and agent_as_handoff, propagated end-to-end through events (AgentEndCall, AgentTransferCall) to harness output types (EndCallOutput, TransferOutput). Defaults to True.
• Convert transfer_call from a plain @passthrough_tool function to a TransferCallTool class (matching the EndCallTool pattern), supporting message and interruptible configuration via both constructor and callable syntax.
• Update _normalize_tools to handle TransferCallTool instances instead of incorrectly treating them as plain callables.

Showing end call being interrupted

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Other: ___________

Testing

• Verify all 32 tests in test_llm_agent_tools_system.py pass (10 existing transfer_call tests updated, 14 new tests for interruptible/message behavior, 2 new _normalize_tools tests)
• Verify TestEndCallAndTransferCallMapping tests in test_voice_agent_app.py pass (4 tests for event-to-output interruptible forwarding)
• Manually test end_call(interruptible=False) and transfer_call(interruptible=False) in a live call to confirm the harness receives interruptible: false

Checklist

• I have read the contributing guidelines
• I have added tests that prove my fix is effective or that my feature works
• I have formatted my code with make format

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Note

Medium Risk
Touches core tool/event wiring and websocket output schemas for call control, so regressions could affect transfer/hangup behavior even though defaults preserve existing behavior.

Overview
Adds an end-to-end interruptible flag for end_call and transfer_call, threading it from tool configuration through AgentEndCall/AgentTransferCall events into harness websocket outputs (EndCallOutput, TransferOutput) and updating logging/mapping accordingly.

Refactors transfer_call from a @passthrough_tool function into a TransferCallTool class (matching EndCallTool), where optional pre-transfer speech is fixed at tool construction (transfer_call(message=...)) and no longer provided by the model at call time; tool normalization/validation is updated to treat TransferCallTool as a first-class tool type. Docs/examples and tests are updated/added to cover the new configuration and propagation behavior.

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