refactor(form): model links as fields

[rekram1-node]

Summary

• model URL requests as external fields within the shared form schema
• support opening, copying, manually completing, and acknowledging external fields in the TUI
• require external fields to be acknowledged before form submission
• require forms and question tool calls to contain at least one field
• immediately accept MCP form elicitations with empty schemas instead of creating empty UI state
• regenerate client types and update form fixtures and contract coverage

Testing

• bun test test/mcp.test.ts test/form.test.ts test/tool-question.test.ts in packages/core
• bun test test/contract-hygiene.test.ts in packages/schema
• bun test test/cli/tui/data.test.tsx test/cli/tui/form.test.tsx test/cli/cmd/tui/notifications.test.ts in packages/tui
• bun test in packages/client
• bun typecheck in packages/core, packages/schema, packages/protocol, packages/server, packages/client, packages/tui, and packages/sdk/js
• bun turbo typecheck --concurrency=3