fix: track concurrent streaming tool calls

[CorgiBoyG]

Summary

• track every active call when a live response invokes the same streaming tool concurrently
• give each input-streaming call its own LiveRequestQueue and broadcast live input to all active call streams
• cancel and release all matching tasks on stop_streaming, transfer, and live-run teardown while preserving later registrations

Fixes #7058

Problem

_process_function_live_helper() stored one task per tool name in InvocationContext.active_streaming_tools. When one live model response invoked the same async-generator tool more than once, each registration replaced the previous task reference. The earlier call kept running but was no longer reachable by stop_streaming() or run teardown, so it could continue writing responses after its agent run ended.

This violated the lifecycle invariant that every background tool call started by a live run remains tracked until it completes or the run stops it.

Solution

ActiveStreamingTool now maintains a private task-to-stream registry while preserving its existing task and stream compatibility fields. Compatibility construction through ActiveStreamingTool(task=..., stream=...) initializes the same registry.

Each call gets an independent input queue. Completion callbacks release task and stream references, stop_streaming operates on a snapshot so it cannot remove calls registered later, and transfer/run teardown enumerate every active task.

The implementation is based on the current live runtime layout:

• google.adk.live._active_streaming_tool
• google.adk.flows.llm_flows.tools._caller
• the existing google.adk.agents.active_streaming_tool compatibility re-export

Testing

• Deterministic end-to-end reproduction through Agent and Runner.run_live:
  • current main at d57c84f1: two calls start; teardown reports [False, True]
  • this PR: two calls start; teardown reports [True, True]
• Covered run teardown and stop_streaming cancellation paths.
• Covered distinct per-call input streams, natural completion, error cleanup, compatibility construction, and preserving registrations created after a stop snapshot.
• 983 passed across the complete LLM-flow, streaming, active-streaming-tool, and FunctionTool test sets.
• The critical concurrency matrix passed 30 repeated runs with PYTHONASYNCIODEBUG=1.
• All pre-commit hooks applicable to the changed files passed.
• Mypy reports no issues in the three changed live/task-tracking source modules.

The full tests/unittests run reached 72% without failures, then stopped making progress for approximately eight minutes and was terminated. It is not reported as a passing full-suite run.

Risk

The change is limited to live async-generator tool bookkeeping and input-stream injection. Existing name-based stop_streaming semantics remain intact: it stops all active calls for that tool name. Public compatibility fields and the compatibility import preserve their existing roles, while completed tasks and streams are released promptly.

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