Fixes an IntelliJ UI freeze when switching between external ACP agent…

[Ju8z]

…s such as OpenCode and Codex in the ProxyAI agent runtime selector.

Problem:
closeSession() was called synchronously from the runtime selector action path. Closing an active ACP session can block while closing stdio transport streams. In practice this can park the Swing UI thread in AcpProtocolCore.close() / StdioTransport.close(), leaving IntelliJ black-screened until the child ACP
process is killed externally.

A freeze report captured the EDT stuck under:

• AgentModelComboBoxAction.actionPerformed
• ExternalAcpAgentService.closeSession
• AcpProcessState.close
• AcpProtocolCore.close
• StdioTransport.close
• FileDescriptor.close0

Changes:

• Close removed ACP process states asynchronously on the service IO scope.
• Remove the session setup mutex immediately after removing the state.
• Stop the child ACP process before closing the protocol.
• Force-kill the child process if it does not exit shortly after destroy().
• Log cleanup failures instead of propagating them back through the UI action path.

Test-Locally

• Reproduced locally with OpenCode -> Codex switching causing IntelliJ to freeze.
• Verified the same close-order patch locally stopped the freeze by letting the ACP process terminate before protocol cleanup.