Fix MCP not loaded on GitHub Copilot CLI

[setoelkahfi]

Related Issue

Fixes #55

Description

Type of Change

• Bug fix
• New feature
• Refactor (no functional change)
• Documentation update
• CI / tooling change

How Has This Been Tested?

• cargo test --all-features
• Manual testing (smb <command>)

Checklist

• cargo check --workspace passes
• cargo clippy --workspace --tests -- -D warnings passes
• cargo fmt --all -- --check passes
• No new unwrap() or expect() calls in production code
• No new mod.rs files introduced
• New dependencies added to root Cargo.toml and inherited with { workspace = true }
• Error messages are user-friendly and surfaced to the terminal

Release Notes

[coderabbitai]

Important

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[sigit-code-review]

This PR fixes MCP server startup on GitHub Copilot CLI by handling the legacy server/discover probe that Copilot CLI sends before initialize. When that probe has no _meta field (i.e. the tool-cache cold-start probe from older Copilot CLI versions), compatible_stdio() responds with a JSON-RPC -32601 error and then hands the remaining stdin back to rmcp. When _meta is present (the modern 2026-07-28 protocol variant), the line is prepended back to stdin and passed through unchanged. The change also upgrades the rmcp dependency from 2.2.0 → 3.4.1 and renames ServerInfo → ServerConfig throughout, with the transport::stdio import removed since rmcp 3.x accepts a (AsyncRead, AsyncWrite) tuple directly.

The core logic to review is compatible_stdio() and legacy_discovery_response() in crates/xcrs/src/mcp.rs.

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