fix: validate MCP allowed tool names at runtime

[mohammedmessaoudene-cmd]

Summary

• validate allowed_tool_names before constructing the MCP protobuf
• reject a plain string instead of silently treating each character as a tool name
• reject non-list containers and non-string list members with stable TypeError messages
• preserve valid lists and the documented empty-list behavior

Problem

The helper is annotated as accepting list[str] | None, but protobuf construction accepts any iterable. Passing allowed_tool_names="safe" therefore succeeds and serializes four entries: s, a, f, and e, instead of rejecting the invalid Python input.

Validation

• targeted runtime-validation tests: 8 passed
• full SDK test suite: 839 passed
• Ruff format/check: passed
• Pyright: 0 errors

Scope

This patch changes only Python-side input validation. It does not change the protobuf schema or the documented behavior that an omitted or empty allowlist permits all tools.