test: add comprehensive tests for Fastly logger to improve patch coverage

- Add tests for Fastly adapter logger initialization
- Add tests for createFastlyLogger error handling and fallback behavior
- Test all log levels and data normalization/enrichment in Fastly logger
- Verify graceful handling of fastly:logger import failures
- Test fallback to console.log when no loggers configured

These tests improve patch coverage by testing all accessible code paths
in the Fastly logger implementation that can be tested in Node.js.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Signed-off-by: Lars Trieloff <lars@trieloff.net>

Author: claude