test: cover websocket plugin behavior

[xingxi0614-cpu]

/claim #71

Summary:

• Added focused Vitest coverage for WebSocketPlugin route registration and configurable prefixes.
• Covered non-websocket upgrade rejection without creating a WebSocket pair.
• Covered WebSocket query messages invoking operations.executeQuery with isRaw: false, the active data source, config, SQL, and params.
• Covered unrelated message actions being ignored and sendMessage delegating to the client socket.

Verification / demo:

• pnpm vitest --run plugins/websocket/index.test.ts -> 1 file passed, 6 tests passed
• pnpm exec prettier --check plugins/websocket/index.test.ts -> all matched files use Prettier code style
• git diff --check -> no whitespace errors

No production behavior was changed; this is intentionally scoped to plugins/websocket/index.test.ts and is separate from the active Durable Object WebSocket coverage slice.

AI-assisted with Codex; I reviewed the diff and kept the PR to this focused plugin coverage gap.