[Agents] replace deprecated AgentNamespace type with DurableObjectNamespace

[atlasgong]

Summary

The AgentNamespace type has been deprecated:

https://github.com/cloudflare/agents/blob/efd12f65ed782a2bc85e75b4f42ea14b599fc987/packages/agents/src/index.ts#L1630-L1636

This PR replaces all references to AgentNamespace in the Calling Agents with DurableObjectNamespace.

The imports were removed as DurableObjectNamespace is a type included in theworker-configuration.d.ts file generated with wrangler types.

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[atlasgong]

I realize that there are other docs still referencing AgentNamespace:

• HTTP and Server-Sent Events
• base prompt
• Build Agents on Cloudflare
• Handle OAuth with MCP servers
• Run Workflows
• Connect to an MCP server
• McpClient — API reference

Is it alright If I replace all references in this same PR?

[mattzcarey]

Hey please rebase since #27273 was merged.

[atlasgong]

Apologies; did not see the existing PR. Seems like there's no extra changes to make! Thanks :)