3/ Add support for HTTP Streamable MCP Transport

[jmorrell-cloudflare]

Note: Stacked on top of #200 *

This adds support for the new Streamable HTTP transport alongside the existing SSE transport. No code changes on the client should be necessary, and you can serve both at once with the same codebase.

This only supports POST requests for now. Resumability et al will come later, which should also not require any further changes to user code.

Setting up a worker to serve these transports on different endpoints would look like this:

export default {
	fetch(request: Request, env: Env, ctx: ExecutionContext) {
		let url = new URL(request.url);
		let path = url.pathname;
		if (path.startsWith('/sse')) {
			return MyMcpAgent.serveSSE('/sse').fetch(request, env, ctx);
		}
		if (path.startsWith('/mcp')) {
			return MyMcpAgent.serve('/mcp').fetch(request, env, ctx);
		}
	},
};

Initial testing with the unreleased MCP Inspector branch seems to work well!

• Not sure if you can set up a stacked diff on github when both branches are remote so that you can only see the second set of changes here? Please let me know the secret it is

[changeset-bot]

🦋 Changeset detected

Latest commit: fa36b3f

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

This PR includes changesets to release 2 packages

Name	Type
agents	Patch
hono-agents	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[threepointone]

this is breaking types in the /mcp example. let's pair and figure it out.

[threepointone]

passed lints/typechecks, now running tests in CI too

[threepointone]

The tests pass, but seem to log this workerd error. Wonder what that's about

workerd/io/worker.c++:2097: info: uncaught exception; source = Uncaught; stack = Error: /worker.ts changed, invalidating this Durable Object. Please retry the `DurableObjectStub#fetch()` call.
    at home/runner/work/agents/agents/node_modules/@cloudflare/vitest-pool-workers/dist/worker/lib/cloudflare/test-internal.mjs:508:15
workerd/jsg/exception.c++:140: info: Annotating with brokenness; internalMessage = jsg.Error: /worker.ts changed, invalidating this Durable Object. Please retry the `DurableObjectStub#fetch()` call.; brokennessReason = broken.inputGateBroken