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/Makefile

@@ -6,12 +6,12 @@
 # Build SeaweedFS binary if not exists
 build:
 	@echo "Building SeaweedFS binary..."
-	@cd ../../../ && make build
-	@test -f ../../../weed || (echo "SeaweedFS binary not found" && exit 1)
+	@cd ../../../ && make install
+	@which weed > /dev/null 2>&1 || (echo "SeaweedFS binary not found in PATH" && exit 1)
 
 # Check dependencies
 check-deps: build
-	@test -f ../../../weed || (echo "SeaweedFS binary not found" && exit 1)
+	@which weed > /dev/null 2>&1 || (echo "SeaweedFS binary not found in PATH" && exit 1)
 	@echo "✅ Dependencies available"
 
 # Run all tests (auto-manages weed mini server via TestMain)