Add .env to the global gitignore, as well as .env.example to python client

[a-akimov]

Motivation and Context

Some basic chores for convenience and consistency:

1. Add .env.example to the python client folder, as it's a good practice and also as a typescript client already has it
2. Add .env to the .gitignore at the root level to prevent accidental commits of this key

How Has This Been Tested?

Yes.

Breaking Changes

No.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

N/a