Complete S3 integration test modernization

- Implement TestMain-based auto server management for all 13 S3 test suites
- Fix import paths to use full module names instead of relative imports
- Update Makefiles to use correct build targets and binary detection
- Enhance testutil with improved binary discovery and credential handling
- Fix test session creation to use explicit AWS credentials
- Validate infrastructure with successful test execution
- Update documentation with implementation details and usage instructions

All S3 test suites now support:
- make test-with-server: Auto-managed server lifecycle
- make test-external: Use existing server (for CI/CD)
- Consistent build and dependency checking
- Proper error handling and logging

test/s3/basic/weed-test.pid

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