FromMCP (OperationAdapter, feature-gated behind mcp) discovers remote MCP
tools over streamable HTTP via rmcp's StreamableHttpClientTransport, calls
tools/list, and registers each as a HandlerRegistration bundle with a
forwarding handler that calls the remote tool via tools/call. Output
handling follows the structuredContent-preferred-over-content-blocks rule:
declared outputSchema + structuredContent is the composable result; absent
outputSchema falls back to the MCP ContentBlock union. isError:true maps to
a CallError with the error content. No-env-vars invariant: the handler reads
context.capabilities (injected at registration), never std::env::var (ADR-014).
Streamable HTTP only — stdio is not built (ADR-037). Provenance is FromMCP
(leaf: composition_authority None, scoped_env None, Internal by default,
ADR-015/022). Includes unit tests for schema/mapping logic and an integration
test that spins up a real rmcp streamable HTTP server and exercises the
forwarding handler end-to-end.
