feat: Implement MCP Streamable HTTP Support in same manner as SSE & stdio

[taylorwilsdon]

The existing pinned version of the client SDK already supports Streamable HTTP transport—this PR simply exposes that capability in our extension alongside the existing stdio and SSE options.

Other Existing Feature Requests / Issues:
#2131
#3627

PR ready @ #4210

---

✨ What’s new / Why it matters

• New transport type streamable-http is now accepted in workspace settings and validated just like stdio and sse.

• Runtime parity – McpHub can create, monitor, and recover Streamable HTTP connections with the same UX feedback you’re used to for SSE and stdio.

• Zero-config upgrades – projects already running an MCP server in Streamable HTTP mode can adopt this feature by adding

  type: streamable-http
  url: http://localhost:8080/mcp/
  

  to their server block—no extension upgrade to the server itself is needed.

---

Implementation highlights

Area	Summary
Schema validation	ServerConfigSchema now recognises streamable-http and enforces the presence of url/headers while forbidding command/args/env.
Runtime wiring	New branch in McpHub.connect() instantiates StreamableHTTPClientTransport, wires onerror/onclose, and updates connection status handling.
DX improvements	Clear error messages for mixed or missing fields, and explicit guidance to set type when a url is provided.

+ import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js"
...
- const typeErrorMessage = "Server type must be either 'stdio' or 'sse'"
+ const typeErrorMessage = "Server type must be 'stdio', 'sse', or 'streamable-http'"
...
- transport: StdioClientTransport | SSEClientTransport
+ transport: StdioClientTransport | SSEClientTransport | StreamableHTTPClientTransport
...
+   } else if (configInjected.type === "streamable-http") {
+       transport = new StreamableHTTPClientTransport(new URL(configInjected.url), { … })
+       …
+   }

---

Acceptance criteria

• Users can define a server with type: streamable-http and successfully connect.

• Status badge in the sidebar reflects connected / disconnected state changes.

• Validation errors surface meaningful messages for bad configs (mixed fields, missing type, etc.).

• Unit tests cover:

  • happy-path connection

  • mixed-field rejection

  • missing-field rejection

• README & sample settings.json updated.

---

Migration / Breaking changes

None—existing stdio and SSE configurations remain untouched.

---

Follow-ups

• Add an end-to-end test in CI that spins up fastmcp --transport streamable-http and verifies a simple tool call.

• Consider exposing retry/back-off settings for Streamable HTTP just like ReconnectingEventSource for SSE.

Let me know if you’d like any additional docs or examples included—happy to iterate!