feat(mcp): switch anonymous sign-in nudge to elicitation

[fahreddinozcan]

Summary

Replaces the in-tool-result sign-in nudge introduced in #2604 with an MCP elicitation/create form request. The previous markdown block instructed the assistant to relay the message to the user and offer to run the setup command, which some agents (notably Claude Code) flagged as prompt injection on the tool result. Elicitations are delivered out-of-band to the client UI, so they bypass that path entirely.

What changes

• maybeElicitAuthSignIn(server, ctx) is called from resolve-library-id and query-docs after the response is built. The tool result text is now clean (no markdown nudge).
• Fires only when the backend has set ctx.shouldPrompt via X-Context7-Auth-Prompt: 1 and the caller is anonymous — same trigger condition as before, just a different surface.
• Gated on getClientCapabilities().elicitation. Clients that don't advertise elicitation see no nudge — safe no-op.
• The dialog presents a single-select radio with two options:
  • "I'll run the command to sign in" — user copies the npx ctx7 setup ... command from the dialog and runs it in their terminal.
  • "Continue anonymously with smaller limits" — suppresses further nudges for the lifetime of the MCP process, keyed per sessionId / clientIp. Decline / Cancel produce the same suppression.
• The command itself is shown in the dialog message; the server does not attempt to drive the client to execute it (no agent-side text injection of any kind).
• Fire-and-forget — wrapped in void ... .catch(...). Never blocks or alters the surrounding tool response.

Deployment dependency

This PR is dormant until the backend re-enables the prompt header. The corresponding one-line flip lives in context7app at lib/api/v2/anonymousSignInPrompt.ts (set ENABLED = true). Without that, ctx.shouldPrompt is never set and maybeElicitAuthSignIn is a no-op.

Known limitations

• The MCP protocol fixes the response action enum at accept / decline / cancel and gives the client full control over button labels. There is no way for the server to rename Accept/Decline to "Close" or "OK", or to render a single-button dialog (spec ref). The two-option radio mitigates the confusion because the user has already chosen intent before clicking Accept.
• "Continue anonymously" suppression is in-memory and resets on editor reload (each reload starts a fresh MCP child process). Durable cross-session suppression would require a new backend endpoint — punted as a follow-up.

Test plan

• pnpm exec tsc --noEmit in packages/mcp — passes
• pnpm exec vitest run in packages/mcp — 12/12 passes
• Manual: point Claude Code at a local build with ENABLED = true in the backend, make 5 anonymous calls, confirm elicitation dialog appears with the npx ctx7 setup command and the two-option radio
• Manual: confirm tool result text contains no markdown nudge
• Manual: confirm picking "Continue anonymously" + Accept suppresses subsequent dialogs in the same session
• Manual: confirm Decline / Cancel also suppress subsequent dialogs in the same session