feat(cli): expand mcp_dart_cli into a debugging toolkit

[leehack]

Feature Description

Extend mcp_dart_cli with debugging and interoperability commands for MCP development.

Motivation

Developers need quick CLI tools to inspect, validate, proxy, and replay MCP traffic without writing custom scripts.

Proposed Solution

Consider splitting implementation into smaller PRs/sub-issues for:

• proxy: stdio ↔ Streamable HTTP bridge
• validate: protocol compliance checks
• call-tool and list-tools
• auth-test
• replay
• host config generation for Claude Desktop / VS Code / Cursor

Acceptance Criteria

• A scoped first set of CLI commands is documented before implementation.
• Commands have help text and examples.
• New behavior has regression tests or scripted smoke tests.

Alternatives Considered

Keep the CLI narrow and provide recipes/scripts in docs instead.