refactor(mcp): client manager hooks into standard AgentLifecycle

[mattzcarey]

Summary

• add an internal, ordered Agent component lifecycle with onStart, onRequest, onTurn, and onDestroy
• make MCPClientManager own its Agent integration: schema setup, HTTP/RPC restoration, OAuth callback interception, tool contribution, state publication, observability forwarding, and cleanup
• reduce Agent's MCP implementation to lifecycle dispatch plus compatibility delegators for addMcpServer(), removeMcpServer(), and getMcpServers()
• route Think's MCP tools and AIChatAgent's MCP readiness through the standard turn lifecycle
• preserve existing MCP storage rows, Agent APIs, protocol frames, React onMcpUpdate, OAuth-provider overrides, and connection behavior

API

Most Agents continue using the default this.mcp without configuration. A custom manager is now installed as a normal Agent component field:

import { Agent } from "agents";
import { MCPClientManager } from "agents/mcp/client";

export class MyAgent extends Agent<Env> {
  override mcp = new MCPClientManager(this, {
    name: "my-agent",
    version: "1.0.0"
  });
}

The previous standalone storage-only constructor, new MCPClientManager(name, version, { storage }), is removed. Keeping one Agent-owned construction mode avoids constructor discrimination, manual host attachment, fallback OAuth wiring, and a second lifecycle model.

Lifecycle behavior

• components are resolved lazily so subclass field initialization can replace the default manager
• startup runs components in order
• request handling stops at the first component response
• turn tools merge in component order
• explicit destruction runs components in reverse order
• a component failure stops the current lifecycle phase

The lifecycle remains internal in this first stage. This PR does not add a raw Durable Object harness, a new MCP package, or a public lifecycle export.

Verification

• pnpm --filter agents test:workers
• pnpm --filter @cloudflare/ai-chat test
• pnpm --filter @cloudflare/think test
• pnpm run build
• pnpm run check
• CI=true pnpm exec nx affected -t test

All checks pass on top of origin/main at a43df13b.

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🦋 Changeset detected

Latest commit: d3e3085

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
agents	Minor
@cloudflare/agent-think	Patch

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commit: ab6b7a5

[cjol]

I think the lifecycle hooks are great and work well here. I'm not entirely sure what each represents yet (what's a request vs a turn? Doesn't matter right now, and I know we'll obviously document them prominently eventually) but it feels a good shape.

I'm not so sure about the interface of MCPClientManager but -- a bit like my state PR from yesterday, maybe it's still progress even if it's not the best shape yet.

[cjol]

I want to think about this for a little bit. Maybe it's the right move, but it makes the interface contract between MCPClientManager and host agent implicit and invites tighter coupling than I'd like.

[cjol]

(same point as my other comment, really) This is effectively the object that we're requiring non-Agent hosts to implement in order to use our MCP client, which is quite extensive and also non-specific (e.g. what bindings need to be returned by getEnv?)

Equally I know from experience trying it with Channels (and then State, where I ended up with a similar situation), that this is the bit where extraction from the current spaghetti gets very hairy, and it's what made me want to go in the opposite direction with Channels -- build a neat, isolated interface for the module, and then put all the adapter code inside Agent rather than bending the module to suit the current Agent implementation.