fix(cli): harden mcp add configuration

[wgqqqqq]

Summary

• reject duplicate MCP server IDs instead of overwriting existing configuration
• parse local MCP commands with platform-aware quoting rules
• require an explicit server type in non-interactive mode
• add focused unit and CLI contract coverage for all three cases

Verification

• pnpm run fmt:rs
• cargo check -p bitfun-cli
• cargo test -p bitfun-cli
• git diff --cached --check

Fixes follow-up issues found while reviewing #2064.