fix(contrib): pool and idle-evict MCP connections in google_adk_agents

[wankhede04]

What was changed

TemporalMcpToolSetProvider's {name}-list-tools and {name}-call-tool activities in temporalio/contrib/google_adk_agents/_mcp.py now pool and reuse a single McpToolset connection per activity name, instead of constructing a brand-new one on every single invocation.

• Added _ConnectionRecord + a module-level connection pool keyed by activity name.
• get_connection() reuses an existing live connection (refcounted) or lazily opens a new one via the existing toolset_factory.
• Idle eviction: once no calls are in flight, a timer (default 5 minutes) closes and evicts the connection. Configurable via a new mcp_connection_idle_timeout parameter on TemporalMcpToolSetProvider.
• Error-triggered eviction: if get_tools()/run_async() raises, the connection is evicted immediately so the next call gets a fresh one. A "no matching tool" business-logic error does not evict a healthy connection.
• Added regression tests in tests/contrib/google_adk_agents/test_mcp_pool.py covering connection reuse, list-tools/call-tool sharing a connection, idle eviction, in-flight calls blocking eviction, and error eviction.

Why?

Every activity invocation called self._toolset_factory(args.factory_argument), constructing a new ADK McpToolset (and therefore a new MCPSessionManager) with no .close()/cleanup ever called on it. For stdio-transport MCP servers this spawns a new child process on every single activity execution and never cleans it up — a genuine subprocess leak under sustained load, not just a missed optimization.

This brings google_adk_agents to parity with the pooling already implemented in the strands (_temporal_mcp_client.py) and google_genai (_mcp.py) contribs, both of which pool and idle-evict connections the same way.

How tested

• Added 7 unit tests in tests/contrib/google_adk_agents/test_mcp_pool.py exercising the pool directly against a fake McpToolset (no real MCP server needed): connection reuse across N calls, list-tools/call-tool sharing one connection, idle eviction after timeout, in-flight calls blocking premature eviction, and eviction-then-reconnect on both get_tools() and run_async() failures.
• Ran ruff check / ruff format --check and mypy --namespace-packages --check-untyped-defs against the changed file — clean.
• Ran the existing non-integration tests in tests/contrib/google_adk_agents/test_google_adk_agents.py to confirm no regressions.

Risks

• factory_argument is only consulted the first time a connection opens for a given activity name; a warm connection is reused regardless of later calls' factory_argument values. This matches how strands/google_genai pool a single connection per name (they have no per-call factory_argument concept at all), but is a behavior change if any caller relied on factory_argument selecting a different backend on every call for the same provider name.
• No worker-shutdown drain hook is added — checking the strands/google_genai implementations directly, neither actually wires one in either (despite that being suggested in the tracking issue); idle + error eviction is the actual existing pattern, which this PR now matches.

Closes #1663

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