feat: add Streamable HTTP MCP transport (closes #62)

[shirshanka]

Problem

Users trying to connect a Streamable HTTP MCP server (the transport used by Cursor, Claude Desktop, and the modern MCP spec) hit three silent failures:

1. No UI path — GenericMcpForm only showed stdio and SSE tabs
2. API rejection — McpConfigRequest.transport was Literal["http", "sse", "stdio"]; Pydantic rejected "streamable_http" before the request reached platform code
3. Engine misrouting — _build_conn didn't handle the "streamable" alias Cursor/Claude Desktop emit, nor the "type" key used by some tools in place of "transport"

Changes

File	What changed
backend/src/analytics_agent/api/settings.py	Added "streamable_http" to McpConfigRequest.transport literal
backend/src/analytics_agent/engines/mcp/engine.py	_build_conn now accepts "streamable" alias and falls back to "type" key when "transport" is absent
frontend/src/components/Settings/connections/GenericMcpForm.tsx	Added "Remote server (Streamable HTTP)" tab; URL placeholder uses /mcp canonical path
tests/fixtures/echo_mcp_server.py	New — minimal FastMCP test server (echo + add tools) for all three transports
tests/integration/test_mcp_transports.py	New — 6 end-to-end tests that actually start the server subprocess and exercise the full get_tools_async() + tool invocation path
tests/unit/test_mcp_engine.py	New — 7 unit tests for _build_conn routing (streamable_http, http alias, streamable alias, type-key fallback, sse, stdio, default)
tests/unit/test_settings_wire_format.py	Added Pydantic acceptance tests for streamable_http transport

Test coverage

tests/unit/test_mcp_engine.py           7 passed
tests/unit/test_settings_wire_format.py 33 passed  (3 new)
tests/integration/test_mcp_transports.py 6 passed
  - stdio tool discovery + echo/add invocation
  - SSE tool discovery + echo invocation
  - streamable_http tool discovery + echo invocation
  - "http" alias (_build_conn output) accepted by langchain-mcp-adapters
  - Cursor/Claude Desktop "type":"streamable" format works end-to-end

Workaround for existing users (until merged)

If your server also speaks SSE, use transport: "sse" in the meantime.

🤖 Generated with Claude Code