Add `mcpplay doctor` diagnostic command

[gauthierpiarrette]

Problem
When a server fails to connect or behaves unexpectedly, users have limited debugging options beyond --verbose. A dedicated diagnostic command would help users self-serve common issues.

Proposed solution
Add mcpplay doctor server.py that runs a series of checks:

1. Script file exists and is readable
2. Python interpreter found
3. Server process starts without crashing
4. MCP initialize handshake succeeds
5. tools/list returns valid tools
6. Tool schemas are valid JSON Schema
7. Schema compatibility warnings (anyOf/oneOf/allOf issues with known clients)
8. Summary: "3 tools found, 1 schema warning, server healthy"

Output is human-readable with checkmarks/crosses. Exit code 0 if healthy, 1 if problems found.

Alternatives considered
Adding diagnostics to mcpplay run --verbose. A separate command is cleaner — it runs checks and exits, rather than starting the full UI.