Stacked on #3006 because of when I discovered this gap.

Summary

• Fix issue where composite tool steps could not access nested fields from backend tool responses
• Backend HTTP client now checks for StructuredContent before falling back to Content array processing (legacy behavior)
• Add integration tests for structured content access and nested defaultResults
• Document step output format and numeric comparison behavior
• I slightly deviated from the solution proposed in 2994, because I felt namespacing all data behind structuredContent would be verbose/clunky. It also does not match much of the existing documentation.

Fixes #2994

Changes

Core Fix (pkg/vmcp/client/client.go):

• Check result.StructuredContent before processing result.Content array
• If StructuredContent is an object (map[string]any), use it directly as step output
• Fall back to existing Content processing for text/image content

Test Helpers (test/integration/vmcp/helpers/backend.go):

• Add StructuredHandler field to BackendTool for structured content responses
• Add NewBackendToolWithStructuredResponse() constructor
• Update CreateBackendServer to use mcp.NewToolResultStructuredOnly() when appropriate

Integration Tests (test/integration/vmcp/vmcp_integration_test.go):

• TestVMCPServer_StructuredContent: Tests nested field access via {{.steps.X.output.field.nested}}
• TestVMCPServer_DefaultResults_NestedStructure: Tests nested defaultResults fallback

Documentation:

• docs/operator/virtualmcpcompositetooldefinition-guide.md: Added "Step Output Format" and "Numeric Values in Templates" sections
• docs/operator/advanced-workflow-patterns.md: Added "Step Output Format" and "Numeric Comparisons" sections