Merge feat/http-from-mcp: from_mcp adapter (rmcp streamable HTTP, tools/list, structuredContent handling)

Implements FromMCP (feature-gated behind mcp) in src/adapters/from_mcp/: rmcp
StreamableHttpClientTransport connects to MCP endpoint, calls tools/list, builds
HandlerRegistration bundles (provenance FromMCP, leaf, Internal, Mutation,
capabilities=bearer token). Forwarding handler calls client.call_tool, maps
CallToolResult per structuredContent-preferred-over-content-blocks rule (declared
outputSchema → structuredContent; absent → ContentBlock union; no heuristic
JSON.parse; isError→CallError). No-env-vars (reads context.capabilities).
Streamable HTTP only (ADR-037). 19 unit + 5 integration tests.

# Conflicts:
#	crates/alknet-http/src/adapters/mod.rs

crates/alknet-http/src/adapters/mod.rs

@@ -10,4 +10,10 @@
 
 pub mod from_openapi;
 
+#[cfg(feature = "mcp")]
+pub mod from_mcp;
+
 pub use from_openapi::{FromOpenAPI, HttpAuthScheme, HttpServiceConfig, OpenAPISpec};
+
+#[cfg(feature = "mcp")]
+pub use from_mcp::FromMCP;