refactor: use Authorization header for API authentication

[MiniCodeMonkey]

Summary

• Refactored API authentication to use Authorization headers instead of query parameters
• Improves security by following best practices recommended in the Geocodio API documentation
• Eliminates code duplication by centralizing authentication in the _request method

Changes

• Modified _request method to automatically add Authorization header with Bearer token
• Removed redundant api_key parameter initialization from all API methods (geocode, reverse, list operations)
• Made params parameter optional in _request method, defaulting to empty dict when not provided
• Updated all unit tests to expect Authorization header instead of api_key query parameter
• Removed TODO comment about repeated API key handling (now resolved)

Test Plan

Unit Tests (✅ All Passing)

• test_client.py - Client initialization and basic geocoding tests
• test_geocode.py - Forward geocoding with various parameters
• test_reverse.py - Reverse geocoding with various parameters
• test_errors.py - Error handling with proper authentication
• All 48 unit tests passing with 85% code coverage

Test Execution

# All unit tests pass
uv run pytest tests/unit/ -v
# Result: 48 passed in 1.05s

Breaking Changes

None - The public API remains unchanged. This is an internal implementation improvement that maintains backward compatibility.