fix: preserve live resumption transparency

[he-yufeng]

Summary

• stop forcing session_resumption.transparent = True when run_live() reconnects with a saved handle
• keep the caller's original SessionResumptionConfig.transparent value intact
• add a regression test for the default basic session-resumption path

Why

Gemini Developer API supports live session resumption, but not transparent session resumption. The reconnect path was mutating the config from a basic resumption request into transparent resumption, which makes Gemini reject the reconnect.

To verify

• uv run pytest tests/unittests/flows/llm_flows/test_base_llm_flow.py -q
• uv run pyink --check src/google/adk/flows/llm_flows/base_llm_flow.py tests/unittests/flows/llm_flows/test_base_llm_flow.py
• git diff --check

Fixes #5771

[he-yufeng]

Rebased this onto current main and reran the focused checks.

Validation:

PYTHONUTF8=1 .\.venv\Scripts\python.exe -m pytest tests\unittests\flows\llm_flows\test_base_llm_flow.py -q --basetemp .tmp\pytest-5784-rebase
# 31 passed, 1 warning

PYTHONUTF8=1 .\.venv\Scripts\python.exe -m pyink src\google\adk\flows\llm_flows\base_llm_flow.py tests\unittests\flows\llm_flows\test_base_llm_flow.py --check
PYTHONUTF8=1 .\.venv\Scripts\python.exe -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 upstream/main..HEAD