fix(ws): support graceful client shutdown

[Zhang-986]

Summary

• make Start unblock when its context is canceled instead of waiting on select {} forever
• let Close cancel the running client context so callers can stop a started WebSocket client without restarting the process
• make ping and reconnect sleeps context-aware, and skip reconnect after an intentional shutdown
• guard connection state snapshots used by background loops so shutdown is clean under the race detector

This keeps the existing blocking Start shape for compatibility, but it is no longer unbounded: callers can stop it with either context cancellation or Close.

Fixes #204
Refs #190

Tests

• go test ./ws -run TestPingLoopStopsWhenContextCanceled -count=1 (red before fix)
• go test ./ws -run TestStartReturnsWhenContextCanceled -count=1 (red before fix)
• go test ./ws -run TestCloseStopsStartedClient -count=1 (red before fix)
• go test -race ./ws -count=1
• go test ./ws -count=1
• go test ./... -count=1
• git diff --check