fix: preserve Live session id on tool responses

[he-yufeng]

Summary

• preserve the Live session id when handle_function_calls_live() builds the matching tool response event
• keep the same session id when parallel Live tool responses are merged
• add regression coverage for single-tool Live responses and merged response events

Fixes #5702.

To verify

• .\.venv\Scripts\python.exe -m pytest tests\unittests\flows\llm_flows\test_functions_simple.py -q --basetemp .tmp\pytest -p no:cacheprovider
• .\.venv\Scripts\python.exe -m py_compile src\google\adk\flows\llm_flows\functions.py tests\unittests\flows\llm_flows\test_functions_simple.py
• .\.venv\Scripts\python.exe -m pyink --check src\google\adk\flows\llm_flows\functions.py tests\unittests\flows\llm_flows\test_functions_simple.py
• .\.venv\Scripts\python.exe -m isort --check-only src\google\adk\flows\llm_flows\functions.py tests\unittests\flows\llm_flows\test_functions_simple.py

[he-yufeng]

Rebased onto current main after the branch briefly showed behind.

Re-ran the focused local checks:

• .\.venv\Scripts\python.exe -m pytest tests\unittests\flows\llm_flows\test_functions_simple.py -q --basetemp .tmp\pytest -p no:cacheprovider (28 passed)
• .\.venv\Scripts\python.exe -m py_compile src\google\adk\flows\llm_flows\functions.py tests\unittests\flows\llm_flows\test_functions_simple.py
• .\.venv\Scripts\python.exe -m pyink --check src\google\adk\flows\llm_flows\functions.py tests\unittests\flows\llm_flows\test_functions_simple.py
• .\.venv\Scripts\python.exe -m isort --check-only src\google\adk\flows\llm_flows\functions.py tests\unittests\flows\llm_flows\test_functions_simple.py

[xuanyang15]

Hi @he-yufeng, thanks for creating this PR! Since there is already fix for the issue which is under internal review, I will proceed with that one for now.