Fix MCP handshake failure with hosts that probe via server/discover

[aniongithub]

Problem

Installing the latest release and enabling it in the GitHub Copilot CLI fails at startup:

MCP server process exited before completing the MCP initialize handshake (exit code 1);
last stderr: Error: expect initialized request, but received: Some(Request(JsonRpcRequest {
  ... request: CustomRequest(CustomRequest { method: "server/discover", ... }) }))

Root cause

The Copilot CLI (and similar hosts) send an optimistic server/discover request before the standard initialize. A spec-compliant server is expected to reply to the unknown method with -32601 Method not found and stay alive, after which the host falls back to the normal initialize handshake. The runtime logs confirm this:

[rust:mcp::client] server/discover failed before legacy initialize also failed
  {"error":"...code:-32601, message:\"Method not found\"..."}

rmcp's serve_server handshake instead treats the first non-ping request as the initialize request. When it isn't, serve() returns an error, main exits (code 1), and stdin closes before the host's fallback initialize arrives — the client then hits a broken pipe and the server never starts. (mind-map, which uses the Go SDK, tolerates this and works, which is why other servers were fine.)

Fix

Wrap the stdio transport in a small DiscoveryGuard (crates/devcontainer-mcp/src/discovery_guard.rs) that, before initialization, answers any request that isn't initialize/ping with JSON-RPC -32601 Method not found and keeps waiting. ping and initialize pass through untouched, and once initialized the guard is fully transparent. This lets the host's fallback initialize succeed, matching spec-compliant server behavior.

No dependency changes.

Tests

Adds two end-to-end tests that run the real DevContainerMcp server behind DiscoveryGuard over an in-memory transport (mirroring the reference server's test style):

• survives_pre_initialize_discover_probe — replays server/discover → initialize → initialized → tools/list, asserting the probe gets -32601, the server stays alive, and tools list successfully.
• normal_handshake_still_works — a plain handshake is unaffected by the wrapper.

Verification

Built and tested inside the devcontainer:

• cargo test -p devcontainer-mcp → both tests pass
• cargo clippy -p devcontainer-mcp-core -p devcontainer-mcp -- -D warnings → clean
• Manual stdio replay of the Copilot sequence: server/discover returns -32601, then initialize + tools/list return 46 tools, exit 0, empty stderr.