Add Streamable HTTP transport to the MCP host (stdio parity, unauthenticated MVP)

[mertsatilmaz]

Carved out of #96 so the first slice of MCP transport work has a clean, reviewable home.

The MCP host currently supports stdio only (mcp_host.py / mcp_runtime.py, exposed through --mcp-host-target + --mcp-runtime-config since #145). This issue adds Streamable HTTP transport alongside it.

Scope

• Config (mcp_runtime.py): accept transport: streamable_http on a server entry, with url and optional headers. Validate it the same way stdio entries are validated today.
• Host (mcp_host.py): dispatch on server_config.transport to a _connect_streamable_http_server(...) alongside the existing _connect_stdio_server(...). Raise a clear AdapterError for unsupported transports and when the optional MCP SDK doesn't expose the client.
• Behavior parity: stdio stays the default and its behavior is unchanged. HTTP runs must produce the same canonical trace shape (mcp_servers, mcp_tool_calls, mcp_events — mcp_connection_initialized → mcp_tools_discovered → mcp_tool_result → mcp_connection_closed).
• Tests: focused transport-level coverage that doesn't require a live network service (fake/loopback in the spirit of tests/fixtures/mcp_servers/).
• Docs: extend the "MCP host adapter" section of docs/adapters.md with a runtime-config example.
• CHANGELOG: [Unreleased] entry (new user-visible config surface).

Out of scope (stays in #96)

OAuth / authenticated transports, SSE, MCP resources, prompts, sampling, default-deny roots.

Notes

An unauthenticated MVP is the right first step — auth lands in a follow-up so the transport plumbing can be reviewed on its own.

Assigned to @Flames4fun, who built the stdio host runtime (#107) and the CLI wiring (#145).

[Flames4fun]

Thanks for carving this out and for assigning it to me, @mertsatilmaz

I’m on it and have started working through the current stdio implementation and the Streamable HTTP client surface in the MCP SDK.

I’ll keep the PR strictly scoped to #161: runtime config support for streamable_http, transport dispatch in the host, stdio behavior parity, clear adapter errors, focused network-free tests, documentation, and the changelog entry.

OAuth, SSE, resources, prompts, sampling, and the remaining lifecycle work will stay out of this PR as requested.

I’ll also review the transport diff from #160 as reference material, while implementing and validating the change independently against the current codebase and supported MCP SDK range.