Add MISTRAL_ environment variable support with precedence over KOYEB_

[nicoche]

Summary

• Add support for MISTRAL_ environment variables as counterparts to KOYEB_ variables
• MISTRAL_API_TOKEN, MISTRAL_API_HOST, and MISTRAL_REGION now take precedence over KOYEB_ counterparts
• Add helper functions in utils.py for consistent environment variable access
• Update all relevant code, documentation, and examples

Changes Made

Core Changes

• Added get_env_var_mistral_fallback(), get_api_token(), get_api_host(), and get_region() helper functions in koyeb/sandbox/utils.py
• Updated get_api_clients() and get_async_api_clients() to use the new helper functions
• Updated create_deployment_definition() to use get_region() helper
• Updated sandbox.py to use get_api_token() helper and updated all docstrings
• Updated snapshot.py to use get_api_token() helper and updated docstrings

Documentation & Examples

• Updated docs/sandbox.md to mention both MISTRAL_ and KOYEB_ environment variables
• Updated examples/README.md to show both environment variable options
• Updated example files to check for MISTRAL_API_TOKEN first, then KOYEB_API_TOKEN

Behavior

• When both MISTRAL_ and KOYEB_ variables are set, MISTRAL_ takes precedence
• When only KOYEB_ variables are set, they continue to work as before (backward compatible)
• When only MISTRAL_ variables are set, they are used
• Error messages now mention both variable names when neither is set

Testing

• All imports work correctly
• Helper functions tested for precedence behavior
• Integration with API client functions verified
• Backward compatibility maintained

Closes: Request to add MISTRAL_ environment variable support