feat(adapters): from_mcp + to_mcp behind the mcp feature (rmcp 1.8)

from_mcp (src/adapters/from_mcp/):
- tools/list discovery over streamable HTTP; per-tool
  HandlerRegistration (Mutation, Once, FromMCP leaf, Internal;
  ADR-015/022)
- structuredContent-preferred output, ContentBlock-union fallback,
  isError -> MCP_TOOL_ERROR with content blocks as details (ADR-023)
- bearer token flows via capabilities key 'mcp' (ADR-014 no-env-vars)
- 19 unit tests + tests/from_mcp_integration.rs (5 tests vs a real
  rmcp streamable-HTTP MCP server)

to_mcp (src/adapters/to_mcp.rs):
- 4 fixed gateway tools (search/schema/call/batch, ADR-041); Sub ops
  excluded from search and uncallable (MCP is request/response)
- identity survives rmcp framing: bearer_auth_middleware stashes
  Option<Identity> in http::request::Parts extensions, call_tool reads
  it back from RequestContext extensions
- StreamableHttpService nested at /mcp in HttpAdapter's router,
  bearer middleware around it (feature-gated)

Streamable HTTP only (ADR-037): rmcp default-features off, no stdio.
Default build compiles without rmcp (cargo tree: 0 hits).

Verified: cargo test (182 lib default / 218 all-features) + 5 MCP
integration + 10 WS, clippy -D warnings (both), fmt.

Cargo.toml

@@ -60,4 +60,8 @@ tokio-tungstenite = "0.28"
 
 [[test]]
 name = "ws_upgrade_session"
-required-features = ["test-support"]
+required-features = ["test-support"]
+
+[[test]]
+name = "from_mcp_integration"
+required-features = ["mcp"]