test: cover websocket plugin behavior

[Treasure520520]

/claim #71

Summary

Adds a focused, non-overlapping Vitest coverage slice for the WebSocket plugin.

Covered behavior:

• default and custom socket route registration
• non-upgrade requests returning the existing 400 response without opening a socket pair
• WebSocket upgrade responses returning the client socket and accepting the server socket
• query messages calling the injected executeQuery operation with context data and sending the serialized result back
• non-query messages being ignored
• sendMessage() delegating to the provided client socket

This is test-only and does not change production behavior.

Verification

• ../starbasedb/node_modules/.bin/vitest run plugins/websocket/index.test.ts --config ../starbasedb/vitest.config.ts
• ../starbasedb/node_modules/.bin/prettier --check plugins/websocket/index.test.ts
• git diff --check

Note: this worktree reused the existing installed dependencies from the neighboring checkout because this machine does not have global pnpm available.