feat: add end_call as LLM function tool

[san0808]

Adds end_call as a special-cased LLM function tool (similar to transfer_call) so the primary conversation LLM can signal when to hang up => idea is to replace the extra hangup_after_LLMCall inference and increase accuracy

How it works: When the LLM calls end_call, the tool result is fed back and the LLM generates a clean goodbye message before disconnect (tool-result flow).

Changes:

• constants.py — END_CALL_FUNCTION_PREFIX and END_CALL_TOOL_DEFINITION
• helpers/utils.py — Skip filler message for end_call
• llms/tool_call_accumulator.py — Skip pre-call message for end_call
• agent_manager/task_manager.py — Handle end_call in __execute_function_call with tool-result flow