Fix broker auto-spawn when stale non-broker PID file blocks spawn

[SoundBlaster]

Motivation

• Broker proxy could skip spawning a new daemon if ~/.mcpbridge_wrapper/broker.pid contained a live but unrelated PID, causing IDE clients to fail to start MCP while direct mode still worked.
• The existing fast-path trusted os.kill(pid, 0) liveness check without verifying the process identity, which allowed unrelated processes to short-circuit spawn logic.

Description

• Add a daemon identity guard in BrokerProxy._spawn_broker_if_needed: if a live PID is present but _pid_belongs_to_broker(pid) returns False, the proxy now logs a warning and calls _cleanup_broker_files() so normal socket-check / spawn flow proceeds.
• Preserve the existing version-mismatch handling and lock-based spawn serialization; only the PID-file fast-path is hardened.
• Add a new unit test test_live_non_broker_pid_is_treated_as_stale and update related tests to explicitly mock _pid_belongs_to_broker where appropriate to reflect the new logic.

Testing

• Ran PYTHONPATH=src pytest -q tests/unit/test_broker_proxy.py and all tests passed: 57 passed.
• Ran PYTHONPATH=src ruff check src/mcpbridge_wrapper/broker/proxy.py tests/unit/test_broker_proxy.py and linting passed.
• Verified the modified code compiles via python -m py_compile src/mcpbridge_wrapper/broker/proxy.py (success).

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