fix: validate mentor issue URLs

[kunal-9090]

Summary

• validate the oss-dev-new mentor issue URL before starting mentoring
• require a GitHub issues URL with a positive issue number
• show a clear Typer error for missing, invalid, or non-positive issue numbers
• add Typer CLI coverage for valid and invalid mentor inputs

Tests

• uv run pytest tests\cli\test_new_cli.py tests\oss\test_github.py
• uv run ruff check src\oss_dev\cli\app.py tests\cli\test_new_cli.py

Notes

• Full suite: uv run pytest currently reports 54 passed and 3 existing failures in tests\cli\test_oss_commands.py because the legacy Click CLI exits early with a missing Gemini API key before argument validation.

Fixes #28

[Hell1213]

Solid fix. Regex is robust and tests pass. Merging.