Add schema constraints to time server strings

[bmtriet]

Summary

• add maxLength/pattern schema hints for time server timezone inputs
• add a bounded 24-hour time pattern for convert_time
• cover the advertised input schemas and representative pattern examples in tests

Context

This is a small defensive follow-up for #3537. It only adds schema-level constraints for clients that validate tool schemas; existing runtime validation through ZoneInfo/time parsing remains the final check.

Tests

• cd src/time && uv run pyright
• cd src/time && uv run ruff check src/mcp_server_time/server.py test/time_server_test.py
• cd src/time && uv run pytest

[LuuOW]

Technical audit: Verified MCP server implementation for consistency with current SDK patterns.