fix: cache prefixed tools per invocation

[he-yufeng]

Summary

• Cache prefixed tool lists by invocation_id instead of keeping only the last invocation.
• Clear the prefix cache in BaseToolset.close().
• Extend the base toolset tests so inv-1, inv-2, then inv-1 reuses the original inv-1 cache entry.

To verify

• python -m pytest tests\unittests\tools\test_base_toolset.py -q --basetemp=.tmp\pytest-adk-toolset-full2
• python -m pyink --check src\google\adk\tools\base_toolset.py tests\unittests\tools\test_base_toolset.py
• python -m mypy src\google\adk\tools\base_toolset.py
• python -m py_compile src\google\adk\tools\base_toolset.py tests\unittests\tools\test_base_toolset.py
• git diff --check upstream/main..HEAD

Fixes #5721

[sasha-gitg]

closing in favor of #5726