fix: preserve nontransparent live resumption

[he-yufeng]

Summary

Fixes #5675.

BaseLlmFlow.run_live always set session_resumption.transparent = True when reconnecting with a saved live-session handle. That overwrote an explicit transparent=False config, which is the configuration needed for direct Gemini API live-session resumption because GoogleLLM.connect rejects transparent resumption on the Gemini API backend.

This keeps the existing default behavior for reconnects that have no session resumption config yet, but preserves a caller-provided non-transparent config.

Validation

python -m pytest tests/unittests/flows/llm_flows/test_base_llm_flow.py -q
# 29 passed

pyink --check src/google/adk/flows/llm_flows/base_llm_flow.py tests/unittests/flows/llm_flows/test_base_llm_flow.py
isort --check-only src/google/adk/flows/llm_flows/base_llm_flow.py tests/unittests/flows/llm_flows/test_base_llm_flow.py
python -m py_compile src/google/adk/flows/llm_flows/base_llm_flow.py tests/unittests/flows/llm_flows/test_base_llm_flow.py
git diff --check