Add Streamable HTTP transport to the MCP host (stdio parity, unauthenticated MVP) #161

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).