fix(host-cloudflare): route toolkit MCP endpoints

[donmasakayan]

Summary

• Route /mcp/toolkits/:slug through the Cloudflare MCP Durable Object handler.
• Record the targeted MCP resource in session metadata and verified headers.
• Reject reuse of a session on a different MCP resource.
• Keep the existing /mcp route and shared cloud host behavior working.

This fixes toolkit URLs emitted by the toolkit UI, which previously reached the worker but returned a route-not-found response. It also prevents a session created for one toolkit from being replayed against the default server or another toolkit.

Validation

• bun run test -- src/mcp/resource.test.ts (10 passed)
• bun run test -- src/worker.e2e.node.test.ts (12 passed)
• bun run test -- src/mcp/agent-session-durable-object.test.ts (39 passed)
• bun run test -- src/mcp-session.e2e.node.test.ts (4 passed)
• root lint passed
• root typecheck passed
• formatting check passed

[donmasakayan]

Runtime validation update from a self-hosted Cloudflare staging deployment:

• Toolkit URL OAuth discovery and authorization succeeded.
• MCP initialize, notifications/initialized, and tools/list succeeded over separate requests.
• Toolkit tools/list returned Executor core tools including skills, execute, and resume.
• skills({ name: "execute" }) succeeded.
• execute({ code: "return { alive: true };" }) succeeded.
• Read-only Google Calendar calendarList.list and Google Drive about.get calls succeeded through connections assigned to the toolkit.

The backport is deployed on NextFinancial staging. No additional route patch was required after this validation.