fix(mcp): consume auth prompt signal

[tandede]

Summary

• consume the backend sign-in prompt signal once elicitation starts
• prevent later stdio tool calls from reopening the same dialog
• add regression coverage and a patch changeset

Why

The backend sends X-Context7-Auth-Prompt: 1 at most once per MCP session. The server captured that signal in ctx.shouldPrompt but never cleared it, so clients advertising elicitation could reopen the same sign-in dialog after every subsequent tool call.

Test plan

• pnpm lint:check
• pnpm format:check
• pnpm build
• pnpm typecheck
• MCP test suite — 80 passed