MUL-6655 Fix MCode ACP session startup race (ZIC-199) (#7516)

* Fix MCode ACP session startup race

Co-authored-by: multica-agent <github@multica.ai>

* fix(agent): only trust transport signals for MCode startup exit

mcodeSessionStartupExited also matched "process exited" / "broken pipe"
anywhere in the error text. A still-running MCode reports session/new
failures as a JSON-RPC *acpRPCError, and session/new is the call that
launches mcpServers, so a real error such as `failed to start MCP
server "filesystem": process exited with code 1` was rewritten into
"MiniMax Code ACP exited before starting a session; ... upgrade
MiniMax Code". That branch returns a fixed string with no %v, so the
original error was dropped, not just relabelled, pointing the member
at the wrong subsystem.

Drop the text matching and keep the transport-level signals. Real
provider exits already arrive as errMcodeProcessExited via
closeAllPending, and a stdin write racing the exit arrives wrapped as
syscall.EPIPE, so no covered case is lost.

Co-authored-by: multica-agent <github@multica.ai>

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Co-authored-by: multica-agent <github@multica.ai>
Co-authored-by: J <bohan@devv.ai>