fix: stop --mcp alone from triggering the UI keep-alive loop

[JProck21]

Summary

infigraph-mcp was leaking worker processes: passing --mcp alone incorrectly
set ui_enabled = true, which puts the process into a loop { sleep(3600s) }
keep-alive after stdin EOF instead of exiting. Over repeated MCP client
restarts this accumulates orphaned worker processes.

Root cause: ui_enabled was derived from
args.iter().any(|a| a == "--ui" || a.starts_with("--ui=") || a == "--mcp") —
the || a == "--mcp" clause meant bare --mcp (no --ui) still enabled the
UI keep-alive path. Fix removes that clause so ui_enabled only reflects an
actual --ui/--ui= flag.

Also extracted the check into ui_enabled_from(args: &[String]) -> bool so
it's unit-testable in isolation from run().

Test plan

• [x ] cargo test --all passes
• [x ] cargo clippy --all-targets passes
• [x ] Tested manually (describe below)

./target/debug/infigraph-mcp --mcp < /dev/null &
  pid=$!
  sleep 5
  if kill -0 $pid 2>/dev/null; then
    echo "STILL RUNNING after 5s — bug present"
    kill -9 $pid
  else
    wait $pid
    echo "exited on its own, exit code: $?"
  fi

The above exited 0 after the update.

Notes

No behavior change for --ui / --ui=<port> / --mcp --ui combinations; only bare --mcp is affected.