BaseToolset caches prefixed tools by a single invocation ID, breaking multi-agent concurrency

Description

In google.adk.tools.base_toolset.BaseToolset, prefixed tool wrappers are cached using a single instance variable self._cached_invocation_id.

When orchestrating multiple agents concurrently (e.g., via ParallelAgent or custom asyncio.TaskGroup harnesses), concurrent worker coroutines call get_tools(invocation_id="worker_N"). Because self._cached_invocation_id only stores the last caller's ID, concurrent workers constantly overwrite each other's cached ID. This results in a cache miss on every single turn, forcing redundant tool wrapper re-instantiations and causing severe lock contention.

Proposed Solution

Refactor self._cached_prefixed_tools to be a dictionary keyed by invocation_id:

self._cached_prefixed_tools: dict[Optional[str], list[BaseTool]] = {}

When get_tools is called, check if invocation_id in self._cached_prefixed_tools and return the cached list for that specific agent thread. Ensure close() clears the dictionary (self._cached_prefixed_tools.clear()).