proxy: fix data race in /running endpoint and typo in error message (#748)

## Problem

The `/running` endpoint in `listRunningProcessesHandler` reads
`process.state` directly without holding `stateMutex`. Meanwhile,
`swapState()` writes to `process.state` while holding the write lock.
This is a data race flagged by the Go race detector.

Also fixes a minor typo: "processes was in state" → "process was in
state".

## Fix

- `proxymanager.go`: Replace `process.state` with
`process.CurrentState()` which acquires `stateMutex.RLock()` before
reading.
- `process.go`: Fix typo in error message.

## Verification

- `gofmt -l` — clean
- `go test -run "TestProcessGroup_|TestProxyManager_" ./proxy/` — all
pass
- `go test ./proxy/config/... ./proxy/cache/...
./proxy/configwatcher/...` — all pass

proxy/process.go

@@ -304,7 +304,7 @@ func (p *Process) start() error {
 					return fmt.Errorf("process was already starting but wound up in state %v", state)
 				}
 			} else {
-				return fmt.Errorf("processes was in state %v when start() was called", curState)
+				return fmt.Errorf("process was in state %v when start() was called", curState)
 			}
 		} else {
 			return fmt.Errorf("failed to set Process state to starting: current state: %v, error: %v", curState, err)